I haven't used the PicKit3 myself, but I have used the PicKit 2 a lot.
I've got myself hooked on an electronics engineering video blog, and in one of the episodes the author talks about the PicKit2, adding a small snippet about the PicKit 3 at the very end. His basic opinion was that there aren't too many advantages to warrant ditching a PicKit2 for a PicKit 3 and in fact there are some disadvantages.
Here's the link to the specific entry in the blog, in case you're interested:
**broken link removed**
Brian