I typically include a strike cruiser and drop-podding devastators in my marine forces
how I use them depends on my opponent's force for the most part, I look for the most obvious spot to deploy things like AA formations (hydras, falcons, zaap brigades) then try to think where I would put those units if I were trying to dodge a drop, then plot my orbital strike around that location
if my opponent has several small war engines (shadowswords, warhounds, void spinners etc) I will often try to guess where one is being deployed, then I will have the barrage and pod arrive in the same space in an effort to stick 3 blast markers on the unit (barrage+deathwind followed by devs sustaining) and break it, but usually I try to go after as much AA as possible
against an army like marines however I know there is not likely to be much on the ground, in which case I'll often stick the devs into a thunderhawk or take rhinos for them (EpicUK allows you to change your mind before the game with this) and I'll aim for either turn 1 or 3 depending on when my opponent chooses (I never seem to win spaceship rolloffs) if T1 I'll plot the barrage on a garrisonable objective hoping to hit some scouts, alternatively on T3 I'll plonk it on a blitz, either mine or my opponent's, just remember to keep clear of the area first....
Against titans or gargants, I would most likely hold back till T3, then pod onto the opposing blitz, hoping that I've got enough activations remaining to break the blitz guard reaver/gargant with blast markers after the barrage/deathwind/devs have had a go
I particulalry like drop podding EpicUK Dark Angel devastators as their slow firing plasma cannons are teh awesum on a sustain fire (8x AP3+/AT3+ shots!!) but regular devs are almost as handy as their extra range allows them to hit stuff even when they scatter pretty wildly
It really comes down to practice and experience, the more you use them (and are on the recieving end) the more you get a feel for them, also best to view the devs as disposable, but your opponent often has to burn 1-2 activations destroying them...