Annabelle is number 100 for 2012 on the SSA list (a little of 3,000 girls in the US named that) so it's not super popular-- but not unheard of either. On the other hand, Isabella is number 3 with almost 19,000 girls named last year. Adding all the -bel/belle names together, yeah, they're pretty common. I know a couple of under 5 years old Annabels-- most of the young Isabelles I know are more in the 5-10 year old range. I like Annabel-- pretty and very feminine. Arabella has the same feel to me though and it's much less common. So if you are concerned about that, I'd use it instead. Anneliese/Annelise is another name that's similar but not used as much.
I'm around a lot of kids and have yet to meet one named either of those names. I think Annabel is classy and pretty but also a little flowery. I find Annabella to be a little too much for me. That being said, Annabel is one of the top contenders for my soon to be born daughter. I think the name is pretty and really like the nickname potential. I am a bit concerned about the Belle ending. I do not want a Belle or Bella nickname, as I feel those are way overused here. But overall, the full name Annabel has a positive connotation for me and most of the people I mention it to.