1. It's fast. My forums loads faster and my users like it. It was a small forum so I took a risk to convert it. Conversion went fine and all topics are redirected nicely.
2. I like the trophies, points, etc system. The portal add-on rocks. Much easier to set up then IP.Content for my needs.
3. I like the moderation tools! They are freaking slick! I don't say this often and never said it for vB, phpBB3, MyBB and IPB.
4. Editor rocks! I like IPB but I hate their buggy editor.
5. Even after adding like 20 different add-ons? My site loads faster. If I had added that much on IPB it would be crawling.
6. Prefixes are awesome. Was using tag/prefix mod at IPB.
Off course, I feel some features are missing in XF compare to IPB but the other benefits outweigh them.
Looking forward for new releases at XF once lawsuit is settled. Was hesitant a bit for conversion first but after using it I feel the money is well worth spend. Started using XF after 6 months of purchase.