Uhhh, to be a little more constructive than ComMan:
Pets are generally not very powerful, but they do help keep enemies off of your (in PvE), and if you keep your Beast Mastery high, they do meaningful damage. (With a low Beast Mastery, they really are mostly pointless.)
Mesmers are a lot of fun, but definitely trickier than some other classes to play. If you think you are up for it, go for it, but be aware from the start that they just aren't as straightforward as Warriors. Or Monk healers. Or Elementalist blasters. Or Ranger Archers. Or Necromancer Minion Masters. Or, well, nearly any class.
Mesmer can be very powerful in PvP, which many players get into later on, so it could be a fine thing to start with.
Mesmers and Rangers are the two classes that are best at messing up other characters (especially in PvP, but even in PvE, especially for stopping enemy healers). Any Me/Ra or Ra/Me is going to be good at stopping other people from accomplishing what they want to do. That's more or less the essence of the Mesmer-Ranger class combination. (This assumes you are using a bow and some interruptive skills.)
Do consider all the differences between being primary Ranger vs. primary Mesmer (not trying to sway you either way, but it is important): Mesmers get more energy and more energy regeneration. It's often not obvious to first time players, but getting higher maximum energy and more energy regeneration is rare, especially energy regeneration. If you are a primary ranger, you can safely expect to have 75% of the energy regeneration of a primary mesmer pretty much all of the time.
Mesmers, however, get Inspiration, an entire attribute devoted to getting more energy. This is true even for seconary mesmers.
Like energy, armor is pretty immutable. Rangers will almost always take less damage than mesmers. This isn't an exciting difference, but if dying frustrates you, it could be an important difference.
You can only use runes from your primary class. Runes increase your attributes. Very few things in the game increase your attributes (besides leveling up and getting more attribute points, of course). If you expect to use more Ranger or more Mesmer attributes, this is an important consideration.
Any plan you have now is likely to go out the window several times while playing. =) Since Guild Wars provides attribute refunds, this is ok, and you can expirement with other things. But your primary class really is forever (well, until character #2

. So make sure your primary is something you are comfortable playing with for a good long time.