I'm exactly at the opposite side (my main is a mesmer and I'm leveling my guardian alt right now), so this is a pretty interesting topic for me.
I simply love my mesmer and could write a huge post explaining all the great things about the profession (illusions are a very unique mechanic and very fun to play with; they have great underwater skills, a very high survivability even in light armor due to all the illusion and cloak skills; an interesting mix of support skills etc. etc.), but instead I would just like to point out some problems you might face, just so you can make an informed decision.
First of all, they can be a pain to level at the beginning (up to level 40). I always heard people complaining in the mesmer forums, but I never understood that. Huh? I can kill stuff, I can level up, I can progress fine. What's the problem? That was before I rolled my guardian and became aware of how much faster and easier I could clear the same early content. After gaining access to the master traits (sharper images, deceptive evasion, greatsword training, chaotic dampening and illusionary elasticity), things get much more fun and the class starts to shine. So you might be a bit spoiled by playing a Guardian. If you decide to roll a mesmer, hang in there... it starts really slow, but eventually gets quite fun.
Mesmers lack good AoE damage, which is particularly problematic in some parts of the game, specially tagging high end DEs. I guess it won't be such a problem for you, since you can participate in those with your Guardian. But I just wanted to warn you that there will be no "hit whirling wrath and be done with it" type of AoE skill. We can solo single high end foes very well, but trash mobs in high numbers sometimes can be a pain.
Finally, mesmers don't have great mobility. So if walking a lot instead of running bothers you, it can be a problem. That isn't a deal breaker for me, but might annoy some people.
I apologize if I'm painting a grim picture. I love mesmers and actually want you to enjoy them as well, so it would bother me if you started playing with one and gave up early due to one of these things. Don't let the butterflies turn you off... it's a very fun profession!
This is exactly my story, my main is a mesmer and im leveling my guardian which is currently a 30. Mesmer is definitely most of the well-rounded profession compared to the other ones. I actually didn't even notice how "hard" it was to level my mesmer before 40. I really don't think it will pose a big problem for you. AOE is lacking but i compensate it with chaos storm and iberserker. Mobility problem i kinda compensate it with using centaur runes. Mesmer has a lot of utilities and they are super fun to use, decoy, time warp, feedback, nullfield.... you will love it i'm bias but it's the truth!