I played some Battlefield Heroes, and it is trash. Just garbage. Isn't a bit old fashioned to have characters spawn into the middle of a firefight? Halo 2 fixed that, and that was 300 years ago.
They try to ape the aesthetic of TF2, but they really don't get it. The tanks make cartooney "sproing" noises when they fire, and it doesn't seem cute, it just seems like someone went around adding random cartooney details.
I probably would have had more fun if I was playing with some friends, but the interface for making and updating characters is so bad, I can't imagine any of my friends would be willing to waste the time it takes to get set up.