I like Simple Press Forum, and they have a great support forum. I ran into an issue with login conflict though. in version 3.x they use non-standard login hooks, so it will conflict with most (if not all) other login plugins. I was trying to use Ajax login with it but my users couldn't register with it. It returned an alert of "unknown registration response". I had to abandon Ajax Login. I found the User Sidebar Panel (usp) plugin to use. It needed a good amount of reconfiguring, but I was ab;e to make it work well with SPF and my theme still looks great since I dont' have to go to the wp-login page.
I am posting this here only if there is someone else who has experienced or will experience this issue. When I was looking for solutions I could find very little.