The best thing to do is to do a test install with the free version and try to get that working as you want it to work for your site. This should tell you if it will work for you. Make a bunch of test users and try to do the things you want your users to actually be able to do on the site. When you run into issues with your site, come back here and see if you can easily find the answers to your questions using the search system as it is.
Keep a log of everything that you do and all problems that you encounter and which of those problems will require you to either buy a module or pay someone else to fix it. There is a free version of Boonex, yes, but that is a little bit misleading as everyone I've encountered who is running an actual working site with actual participating members has invested hundreds if not thousands of $ into it.
Even better, try to find a copy of an old installation and then do everything above and upgrade the site through several versions.
IF that goes smoothly for you and everything works the way you want/need then it is probably just perfect for you. If it doesn't, then you should probably see what else is available. I think that whether it will work for you is a VERY individual thing based on how adept you are at html, css, php and general troubleshooting as well as how much $ (and time and sanity) you want to invest over the long run.
There has yet to be, from my understanding, a bug free version of the software - and there isn't, from my understanding, ever likely to be one. Every release fixes some bugs and every release postpones bugs to be dealt with until some other date (spend some quality time on the forums and look how many issues go completely unanswered or postponed for a future release and how many issues have either people commenting that they've never had it working properly and how many issues have posts from 6.x versions and still aren't quite working well).
Dolphins' a great tool in many respects, but it is NOT software that you just install and set up quickly and let it run.