For me it's pretty much what Scarf just said.
The best thing is that it's an open franchise. You can always add more to it. More Pokémon, more regions, more stories. There's no inbuilt limit. It's a unique shared imaginitive space that the creators and the fandom can both build on. It's a wonderful example of the possibilities of new media culture.
The worst thing, as Scarf said, is by far the anime. A couple of the movies are pretty well-produced, if improvable, but the anime is really flat and doesn't have much more than nostalgic value. I can think of a couple standout funny episodes (I was watching the episode where James poses as Professor Oak with my partner's four-year-old yesterday, and I think I enjoyed it more than he did), but not enough to redeem it as a whole.
It feels like such a wasted opportunity, given the plots of the games and the comics. Both the visual style and the storylines could be incredible, with what they have to draw on, and it's like they don't even try, even by kids' merchandising show standards. I'd have thought Pokémon would have quite a budget to put toward it by now, if they wanted to.
I'm not grumpy about the anime like I used to be. Like I mentioned, I enjoy episodes sometimes. It just makes me sad, imagining what kind of show the Pokémon anime could be. Especially nowadays, with so many great examples of what a kid's show can be (think Avatar).