I've played almost all limits of online poker, but found my highest ROI in low stakes games.
The reason players advocate playing higher is because the truth is, it requires two completely different skill sets to beat higher games and lower games.
In lower games players don't adhere to sound strategy and as a result, many players have no idea how to go about handling opponents who seem to have no rhyme or reason for their play. While at the higher games, the players do tend to use some type of strategy and it makes them 'seem' easier to play against.
But this is not the case, if you are sitting at a higher buy-in game where all of your opponents understand how to play optimal strategies, even though you know what they are doing, it's going to be much harder to exploit them than it would be to exploit a table full of donkeys who can't fold top pair.
The problem for most players in low-limit games is they fail to properly exploit their opponents because they are too busy trying to play 'correctly'. For example, in some low-limit situations open raising 20x the BB with pocket aces from UTG is the best play or over shoving all-in on a 50 chips pot with your whole stack of 1500. Obviously this is not always the case.
The trick is to learn how to identify these situations and learn to exploit these opponents, once you learn how to do this and see that you can use seemingly 'rediculous' plays to get maximum payouts (and in turn profit) on lower limit players, you will quickly realize why these games are profitable. Because you won't be able to make brash moves like this and exploit opponents in such obvious ways in higher limit games.
Hope that helps a bit.
Also you can view this thread for more information: