I'm going by the Army List Compendium version (3.0.3) of the Gargant Bigmob list, since I expect it will be the main version that players encounter when looking to start an army. However, I'll also be referencing the 3.0.5 version that Evil&Chaos released in 2012, which can be found
here, as a point of comparison.
ConceptThe core thematic concept of the list is 'Gargants and Mad Meks', which is pretty awesome. E&C seemed very determined that this theme should remain as strong as possible.
Unfortunately, I think this gets undermined by a simple weakness of the list: it tends to result in armies with too few activations, because gargants are damned expensive. E&C advocated "a minimum of six activations" at 3000 points, which basically means one Mega-Gargant or Great Gargant and one Gargant, maximum - he's also said that it's basically impossible to build a viable list with both a Mega-Gargant and a Great Gargant.
(In the 3.0.5 version, this is even further enforced by limiting the number of non-gargant formations to three per Gargant/Great Gargant and four per Mega-Gargant. Within that framework, it's effectively compulsory to take one gargant of any size and one regular Gargant, with an absolute maximum of nine activations. I feel that's probably a bit too prescriptive.)
If a list with two big gargants (Great or Mega) is basically a non-starter, I'd like to see the option to build a list focused around one heavily-customised big dude supported by non-gargant formations.
The Mega-GargantSince the other two sizes of gargant are pulled directly from the Ghazghkull list, there's not much to say about them. However, the Mega-Gargant is unique to this list, and therefore a defining feature. In that role, I feel it has some shortcomings.
First, it locks out the other half of the theme - Mad Meks. This thematic element mainly comes through in the Kustom Upgrades - but the Mega-Gargant is not allowed to take any. I assume this was partly because the Mega-Gargant is already loaded up with every weapon under the sun, and partly to stop certain abusive builds, but I feel like this is an unfortunate thematic break that could be avoided with some re-working.
Second, it's surprisingly fragile. It lacks the reinforced armour of its smaller siblings, and is therefore significantly easier to take down than a Great Gargant (except when using TK weapons exclusively). Since it will inevitably be both your Break the Spirit target and your Supreme Commander, this is a big liability, and encourages players to take a Great Gargant instead for the extra resilience, even though it means giving up quite a bit of firepower.
Also, for some weird reason, its Gaze of Mork is weaker than the ones on the smaller gargants - their version is TK, while the Mega-Gargant version is only MW (and FxF as well, while the others aren't).
It seems like a shame - a Mega-Gargant is a perfect centrepiece model for an army, but it's simply a sub-par choice at the moment.
Supa-Stompas vs GargantsI'm not sure why you'd ever run a Gargant instead of two Supa-Stompas. (In the 3.0.5 version, the reason is clear: you're forced to. That's not a good or satisfying reason.)
Two Supa-Stompas are almost exactly as resilient as a Gargant, have double the firepower, and either cost the same (in two formations, with the advantage of an additional activation) or 50 points less (in a single Big formation).
In a list that's meant to showcase gargants, having them lose out so badly to Supa-Stompas seems like a terrible idea.
(Also of note: there's no profile for Supa-Stompas in the 3.0.5 list, even though they're still an option. On the other hand, there are Kustom Stompas from the Stompy Onslaught list, which are still just as resilient, have a bit less firepower, gain a transport capacity, and cost less than half as much. The comparison gets even worse!)
Kustom UpgradesThe other defining feature of the list is the ability to modify the gargants and Supa-Stompas, as well as units of Big Gunz, with Meks. Sadly, most of the upgrades don't seem that useful.
For starters, every upgrade (except the Wyrdboy Tower) demands that you replace an "arm or belly weapon" (and in the Compendium list, it's specifically a Gargant weapon - Supa-Stompas aren't Gargants and don't have Gargant weapons, nor does an Oddboy joining a unit of Big Guns or a Gunfortress). Nowhere is the location of gargant weaponry defined - we can assume that Soopaguns and Lifta-Droppas are arm/belly weapons while Big Guns are not, but that's just a guess. This should be clearly and explicitly defined - "Each Kustom Upgrade replaces one Soopagun, Twin Soopagun, Supa-Zzap-Gun, Lifta-Droppa or Mega-Choppa on the unit with one of the following".
In addition, most of the Kustom Upgrade weapons aren't something that's worth giving up either a 2BP MW or 3+ MW TK(D3) gun to obtain. The Megalobba is particularly egregious - since it has different special rules to the Soopagunz (it's not a MW) they can't be fired together, which means that on Supa-Stompas and gargants it's worse than useless.
So far, most lists I've seen have exclusively used Bigga Generatas for the extra power fields. That seems like a clear indication that something is wrong.
The Big Kustom Upgrades allow smaller gargants to take one of two Mega-Gargant weapons or gain a transport capacity, and everyone picks the Wyrdboy Tower because it's vital in a list with very limited air defense options. Again, having an auto-take option is not a good sign.
I feel like the Transporta upgrade is something that should have been built into all the gargants anyway. All the Ork fluff I can remember has gargants swarming with hordes of Orks (chanting "'Ere we go, 'ere we go," naturally) and charging out when it gets close to the enemy. The same is also true of Stompas, which have a big transport capacity in 40k.
I think the list could easily get away with just having a "Hitch a Ride" special rule which gives all gargants a transport capacity equal to their DC (plus Grots equal to half their DC) for free. This would also allow Mega-Gargants to gain a transport capacity - it's weird that they can't when the smaller gargants can.
(Note that in the 3.0.5 list the Transporta upgrade is almost completely useless, because the Ork War Engine Transports rule prevents all War Engines from transporting any units except those in their own formation.)
I also think it's sad that both kinds of upgrade are limited to 0-1. If someone wanted to go nuts and trick out their gargant with a bunch of kustomisations, would that be so terrible? Doesn't that sound exactly like something a Mad Mek would do?
An interesting alternative, I think, would be to give all the upgrades an appropriate cost, but allow them to be taken in addition to a gargant's existing weapons. If you could take a Great Gargant and add an extra Soopagun, two Supa-Zzap-Guns, a Grot Mega Missile pod, two Kombat Arms and a Wyrdboy Tower for about 200 points, you wouldn't even need a separate Mega-Gargant entry because you basically just built a kustom one! To stop things getting out of hand, perhaps allow each Oddboy (anywhere in the list) to add a kustom upgrade to the Mekboy Big Boss' gargant (with upgrade-specific limitations like 0-1 Wyrdboy Tower, for example) - now you've got a big crazy gargant centrepiece and a bunch of Mad Meks, which sounds bang-on for theme.
The Need for SpeedThe list's major weakness is that it's predominantly pretty slow. Yeah, you can put your Loota Warbands in a Battlefortress if you want them all to die in a massive fireball, or mechanise them in Battlewagonz and such at great expense, but that's about as fast as it gets and you don't really have the points for it.
The Compendium list has the Tellyportas rule, which allows you to Planetfall a formation if you have a spacecraft. Ork spacecraft are just as garbage here as everywhere else, so scratch that. (The 3.0.5 version of the list scraps the rule entirely, making spacecraft even more worthless.)
Any ability to cross the board, then, basically comes from Drop Roks (which have Self-Planetfall). So, um... what are the stats for Drop Roks? In the Compendium, there aren't any. (The 3.0.5 list includes their profile.)
I think it's a bit weird, given E&C's focus on the theme of the list, that Drop Roks are one of its defining, unique features - they just don't seem to fit. Orkstein, in the fluff E&C wrote, pretty much focuses exclusively on gargants. There's no mention of his propensity to seed the low orbits with asteroids filled with Boyz, or how it relates in any way to his favoured tactics.
I think the ability to transport Warbands in gargants (see above) would make it easier to do without crutches like the Drop Roks. You could include upgrades which increased the gargants' transport capacity, and its movement speed. In addition, changing (3.0.5: and reinstating) the Tellyportas rule to allow for teleportation instead of Planetfall (and removing the awful spacecraft from the list altogether) could be quite neat, and very much in keeping with the Mad Mek theme, while becoming very distinct from other Ork lists.
Anyway, those are my main thoughts on the list as it stands. What do other people think?