I've played in two BWE's as a Mesmer/Monk, and only got up to level 10, so I'm not what you'd call an "expert". But Me/Mo was the first character I ever played in GW, and the only character I really played in Post-Searing. So I can speak to it from a newbie perspective ... and I've also played exclusively PvE up to this point.
Basically what people have already said here is true from what I can tell. If you can get used to the fact that the benefits you bring to your team, and the detriment you can bring to the monsters/opponents, are not obvious, then you can enjoy being a Mesmer. The only thing "in-your-face" about them is the uniform ... they are most effective when they are subtle, keeping the balance of power on the side of their team in such a way that a truly good Mesmer would probably never be noticed.
I played a Paladin in EQ and I loved stunning, because I found that making it impossible to hit me was a better way to tank than just standing there and needing heals. The Mesmer brings some of that to GW ... you will be focusing on keeping the other side from doing what they do best. There is also some of the EQ Enchanter in the Mesmer, but not as much as I thought at first ... as someone else said, they don't have the crack or the group haste, nor is their primary function "mesmerizing" in any EQ-familiar sense.
Like I said, I'm not the expert on the class, but I took to the Mesmer pretty quickly and I think pretty well. It wasn't hard at all, once I got into the Post-Searing and really started to learn what I could do well and what I couldn't. I'm probably still a "type 1" right now, but I'm studying to be a "type 2" ... and when I reach that point I expect to be a valuable addition to teams for PvP and PvE.
With that said, I think PvE is a great training ground for the Mesmer, since most of the monsters (as far as I got at least) are pretty much 2- or 3-trick ponies and have limited repertoires to learn how to counter against ... as opposed to jumping into PvP and having to figure out how to counter 8 skills you've never seen before.
My 2 cents.

My suggestion is, if you're interested in a Mesmer, give it a shot! I didn't see nearly enough of us in the BWE's ...