Most MMOs now have some form of free trial. That may or may not have limited functionality; LotRO's did when I tried that, but that was a while back.
But definitely shop around, what you may not think is a good fit for you based on what you hear may well be your perfect game. After all, most people have negative opinions of games they haven't played. You remember negative comments more than positive ones, and after a while the negative comments all pile on each other. After a while, the "Aion/L2/FFXI are super-grindy" and "AoC is 'mature' because it has boobies" comments add up.
I personally greatly enjoy WoW. It's well-supported, very popular, and doesn't believe in punishing the player for choosing to play the game. It certainly didn't hurt that a lot of my friends played it. I actually got pulled into it by having one of those friends show me his character.
I tried Champions Online when several friends started with that, but was irritated by the "character builder that can build anything you want" can't actually build everything I wanted; seriously, is it too hard to include a dinosaur or dragon head? Really? That, plus that it stressed my system, led me to drop it.
Getting back to the popular...a large portion of your experience is going to be social. If you join a game where you don't know anyone, it probably won't be as fun as it would if you do know someone there.
Brad