Batman Beyond is best if it's just taken as an Elseworlds kind of thing, and if you forget about the name "Batman" being strictly tied to "Bruce Wayne". It's a good show, but it's an almost completely different kind of character. Purists probably hate it, but simply taken as an original superhero cartoon, it's fantastic. Honestly, I'm trying to think of any other American-made action cartoons (that aren't directly based on a comic) that are actually good, and almost none come to mind. Beyond is pretty much tops in that regard.
And the sort of "epilogue" they created for it in Justice League Unlimited was brilliant.