I have had positive experiences with bbPress. In previous sites I have used vBulletin as it seems to be the most used commercial forum package out there but I found the customisation very painful (perhaps they have improved it since v4? Their admin was a million miles away from WP).

I ended up moving to bbPress when v2.0 was released (I had tried and dismissed the previous version). Theme compatibility was an issue, but it works well with StudioPress Genesis (thanks to a helper plugin) so that hasn't been a problem for me. And at least this gives you a chance of an integrated user experience, there is no way I would be able to style vBulletin, MyBB, etc to match my blog! Before bbPress I'd have to manage two systems which looked and felt like two different sites.

Setting up is very straightforward imo. You just need to add your forums like anywhere else, the user management aspect is all through WordPress. So Im not sure how any other forum package makes this job easier?!

Customisation has been easy enough, writing a plugin is straightforward if you are familiar with WP plugins. The general concepts are the same (it is based around custom posts after all!) and there are a number of bbPress hooks that have allowed me to do all I need.

The advantage of having it seamlessly integrated into WordPress makes all the difference to me. As a hobby blogger the convenience of having one system to manage is perfect, and it now actually feels like I have one integrated site rather than miss-matched off the shelf products. Sure, it isn't so feature rich but I never used 99% of what eg vBulletin offered me anyway so I really don't miss it! And actually lots of cool additions are available through standard WP plugins! I found other forums eg vBulletin plugins to be very disappointing, I was lead to believe I would be spoilt for choice but they often did work for certain versions or were long abandoned... we really are spoilt at WP.

Hopefully being an Automattic associated plugin means that it will have some longevity and continued investment however my only criticism is the slow pace of development, it's been a long time since there has been a release whilst eg BuddyPress has enjoyed multiple iterations.

Thankfully @JJJ has been funded by a few of us via Indiegogo to dedicate some time to making it better. I hope he makes some good progress and would happily donate again to keep the momentum.