Guardians seem like they're going to be very strong in PvE because their AoE tanking buffs people so strongly. I did AC yesterday and was surprised how much of a breeze it was (after struggling in there on my engineer) just because I was mace and shield AoE supporting allies, people took very little damage. I also was very tanky individually, tanking one of the lovers bosses solo (while my team took down the other) was a walk in the park. I don't know how strong they'll be in PvP yet, I suspect weaker than PvE because there's less times you'll be grouped up and their DPS isn't too strong, but I've not tried a damage build yet, so I might just be underestimating how much damage they can output with the right build or done much PvP at all so might not realise how often there's large team fights (talking proper PvP not WvWvW here).
I've got a level 42 guardian but I've got for a 2h crit build and the damage is great, I can destroy groups in seconds. I have also done some wvw, when it will actually let me in, and it is so good. I can kill anyone really quickly and I'm really tough to kill. Took 8 people to run me down the other day.
The only negative is that the range is awful so you have to play defensively and wait for a small group to break away then jump on the and rip them apart with aoes. My favourite skill though is the reflector shield, I just run at the enemy team drop the shield in front of me and watch them kill themselves with reflected damage



