Running IPB4 here, and I love it! I think XenForo would be somewhat cheaper, and not just because it has no renewal costs, but at least I get great support and neat spam prevention with an active license, so I don't mind paying that $50 a year or what it is.
Sadly compared to IPB3 the mod community is still somewhat behind, but luckily almost everything I want to customize can be done without any mods. So my vote goes to IPB!