Coincidentally, I just started watching Brave and the Bold last night and the first episode I saw was the Owlman episode. At first I wasn't too sure about Jeff Bennett (spelling?) doing the Joker, since I was so used to Hamill, but when I saw how campy and silly the show was, I really enjoyed his performance.
If Jeff had done the Joker for a more serious-toned Batman show, though, I wouldn't have liked it as much.
For a serious-toned show, I do think Hamill hit the nail on the head with his performance. I may just be overly comfortable with his performance because I don't know how I'd react to any other interpretation of the Joker in future cartoons. I wouldn't be against having someone just do a Hamill impression (Hell, if a guy on YouTube can do it nearly spot-on, I'm sure many others can as well), but I wouldn't go berserk if someone wanted to try a new approach to his voice.