The "All-Round Mesmer"

This is my mesmer build that I have used and tried in PvP to great success:

Domination 14
Illusion 11
Fast Casting 8

^ Some of those levels are boosted with runes. I have a superior domination and a minor vigor with a few item boosts putting me into the arena at 435hp with 42 energy points.
I still have points to spare with one 15 point mission to go, so these can be boosted further. Armor is a mix of Rogue's and Enchanter's to get some protection as well as extra power.

Skill Set:

Power Spike
Energy Burn
Backfire
Ethereal Burden
Clumsiness
Empathy
Power Block (Elite)
Rez Signet

With this build, I can take on spellcasters as well as melee warriors and have had to do so 1v1 on numerous occasions and come out victorious. Here's how it works:

Power Spike: This is my instant casting interrupt. Handy if you want to take out a caster that hasn't been hit with backfire. Some people may prefer Power Leak but I choose this because this Mesmer is all about physically hurting other players, not draining energy.
Energy Burn: No mesmer should be without this one. It is one of the few direct damage spells a memser has that takes away HP as well as spell points. Useful for damage against both casters and melee fighters. No self-respecting Mesmer would be using Conjure Phantasm as their direct damage dealer by this stage.
Backfire: Needed for quickly shutting down healers and damage casters. Fast casting is needed to quickly get this off. At 8 Fast Casting, you're quick enough to get off a Backfire on a caster that is casting a 3 second spell.
Ethereal Burden: I prefer this over Imaginary Burden. It doesn't last as long but it's one of the few Domination spells that actually gives you back spell points which is perfect for a Mesmer that hasn't invested in inspiration. At 14 Domination, you make a tidy profit of 4 extra spell points. This spell is perfect for casting on a warrior you need to escape from or a caster that is trying to runaway from your teammates. Also, I pile this on top of Backfire if I think Backfire can be dehexed. This is why Fast Casting is needed.
Clumsiness: I cast this one on a melee fighter that runs up to me thinking I'm easy to kill. They take 70 damage at 14 Domination and almost always, the attacker will get hit by this one. Good for pesky rangers as well.
Empathy: Cast on a melee fighter that is attacking you or on a fighter attacking your teammates. Energy Burn, Empathy and Clumsiness on a warrior attacking you will soon equal a dead warrior.
Power Block: My other instant interrupt with more powerful consequences. Cast this on a healing monk about 20 seconds into a battle and you've shut them down long enough for your team to do major damage. For those that want to lean more towards being anti-melee, Signet Of Midnight may be a decent choice. Up to you.
Rez Signet:For your teammates. If I didn't have a rez, I would invest in Guilt or Shame for energy stealing.

So basically, your toolbox against casters is:
Power Spike
Energy Burn
Backfire
Ethereal Burden
Power Block

Your arsenal against fighters is:
Energy Burn
Ethereal Burden
Clumsiness
Empathy

The key to this build is high Domination making the almost total Domination skill set as powerful as possible. This build is primarliy focused on damage to hit points: you want to put people down. Just today I was attacked by an Illusory Warrior who slowed me down and then hit me with illusory weapon for 41 a hit. An Empathy and a Clumsiness later and that IW was running away, but not before an Energy Burn finished her off! Guess it's back to the drawing board for that one. Warrior Monks suffer a similar fate.

Advantages:
1) Capable of taking on any class yet still be effective.
2) Spells are reasonable in cost.

Weaknesses:
1) No self-healing (pray you have a good team). My logic is that if you are being pounded by 3 warriors, an Inspiration spell won't save you.
2) Easy to burn all your energy if attacked by a melee fighter.