There's always a bit of apprehension when it comes to picking up a new game -- it might be great, or it might be terrible and mean that you're out time and money. Picking up a new MMO on launch day is even more of a gamble, since it could result in your being in on the ground floor of the next big thing... or it could mean wasting a month of playtime on a game you don't like. So it can be tempting to wait for a review of some kind to come in from sources you can trust.
On the one hand, having a review means that you have some idea of what you're getting into before you buy. On the other hand, reviews of MMOs are difficult at the best of times and outright incomplete at the worst -- after all, it can take a long time to reach the endgame and see all the game has to offer, at which point you're no longer in on the ground floor. So do you wait for reviews? How many external opinions do you need or want before you'll commit to a new game?