I have played exclusively with Marines and Eldar for over 10 years, with mixed results to be sure (I am nowhere as successful as Steve or Dave
). I might add that apart from appalling dice, I am also know for my more 'unusual', even adventurous, lists . . .
I should also add that the following really relates to 3K games, which is the main meta for the vast majority of the players. Larger games allow more flexible strategies which in turn permits the use of less usual formations and combinations of units / upgrades.
In answer to the original question, IMO there has been a degree of power-creep in the more recent lists (eg discussions on Gorgons and other newer units), which in turn has caused a review of the original core lists over recent years. Does that mean that Marines (or Orks, IG and Eldar) are now underpowered? Probably not, though as others have noted, players are increasingly having to resort to optimal builds and tactics in order to achieve the same results.
As others have noted, Marines tend to function best with a significant air presence rather than a purely ground-pounding composition, because this gives the greatest flexibility which in turn allows the Marines to gain and retain the initiative. In the UK we also allow the Marine player to revise his transport options for each game, provided he has built this flexibility into the army.
High formation numbers also helps which in turn precludes very high cost formations like termies mounted in Land Raiders, while promoting the use of Land Speeders and Scouts. Terminators, Thawk(s), Tbolts, Scouts and WarHounds are pretty much staple choices. Tacticals with additional upgrades make a useful and robust BTS, though otherwise tend to be increasingly ineffective against newer units and formations.
Personally I prefer Bikes with a Chaplain over Assault Marines; 25 points extra give you one more unit with better stats. I also quite like Devastators with dreadnoughts which make a useful defensive force that can also be podded offensively with a spaceship (esp under uk rules). Equally Devastators mounted in four Razorbacks can make an interesting compromise over Predators. Finally I have been known to field Termies with Dreadnoughts in a Landing Craft for a really awesome assault - though at 850 for two formations, this only comes into its own in larger games.
To be fair, it is also very important to know the strengths and weaknesses of the opposing army, which is also becoming harder with the increasing number of lists. This is an important factor when using an air-assault list because by its very nature this puts a significant portion of the Marine formations in close proximity to the enemy. If the initial assaults fail to destroy their targets, the Marines usually find it very hard to win and are often forced to minimise the losses. I put some, though not all of the 'power-creep' down to ignorance of the opponents capabilities.
Finally I agree with the point about the player's temperament and perspectives. Some players have achieved considerable successes in the past with unusual army builds (eg multiple Whirlwinds). I suspect that this was because opponents had not faced the associated tactics, though IMO this highlights one of the strengths of E:A; the system does allow players to exercise a significant degree of imagination both in preparing their army and its associated strategic intentions, and also in reaction to the particular opposition and events on the battlefield.
