I use Simple Press on one site and it is good but I find it cumbersome to manage and a bit sluggish in performance.
I have recently tried Mingle and it was good and easy to manage but it appears that the developer has all but given up on maintaining it and the support forum has been virtually taken over by someone trying to sell skins.
Because of the above I tried bbPress 2.2.4 and found it easy to use but I spotted a few minor issues. Since then, the 2.3 RC1 has been released (available for download from the bbPress forum site) and these issues have been solved.
Also, 2.3 now includes a number of import templates that enable a simple transfer of data from other forums. I used the one available for Mingle and got a very clean import of all forums, topics and users.