I love vB 3.5, I won't touch v4.. maybe some day I'll upgrade though, but the way i feel now, my forums are going to be continuing to run 3.5 for a long time.
Understand your hesitance to move to vb4, but you should seriously consider upgrading to at least 3.8.6; 3.5.x is old and may be vulnerable to attack.
Actually I don't really have an opinion on 3.x vs. 4.x. Our board only opened in February of this year, so we started off on 4.1 Suite. I'm certainly aware of all the complaints about vb4 though. Internet Brands definitely dropped the ball with the initial vb4 release. IMO 4.1.5 and 4.1.4 are both big steps in the right direction. Not everyone likes the new CKeditor, but apart from that it does seem like they've actually improved vBulletin. vBulletin 3.8.6 is still very stable and secure AFAIK. Given the cost and potential hassle of upgrading I understand why a lot of forum owners choose not to. My advice to you would be if you're happy with your current setup, stick with it. vBulletin 5 is somewhere in the not-too-distant feature, so wait until then to make a move. The way I look at it, either Internet Brands will get it right and release a good update, or they won't, and everyone will convert their forum databases to Xenforo.
I actually paid for version 4, but after testing it and modifying it for more than a couple of weeks, I decided to stay with my 3.8 installation.
I'm still on 3.8. I was really, really close to upgrading to 4.x.....but meh, if it's not broke then don't fix it.