Belle is great. I like it a lot. More and more all the time, actually.
I think Belle, by virtue of its single-syllable tidiness, is more distinctive than Bella, Isabelle, or Isabella, but there's no denying there are a lot of girls with very similar/related names running around. That doesn't even count the Annabels and Arabellas, or all the Ellies and El- starting or -el ending names.
I've never known a Belle, but I have seen it multiple times as a middle name. FWIW, I only like Belle as a fn. It seems like all these short, pretty names end up used over and over as stock middle names, and that just drains them of so much of their charm.
I prefer Belle over Bella, and I would personally choose Isabelle, nn Belle, over just Belle. I love Isabelle, though, so it's a win-win for me and an easy choice. If I didn't love Isabelle, I'd just use Belle. I think it's fine on its own.