One thing important for anyone trying to decide to join a project, is to spend a little time reading the forums and FAQs. And checking the stats of some of the top users. That will give you an idea on the status for the project, what some of the problems might be, and if its the RIGHT one for you. Many projects don't have work, some are application/OS specific and some are not worth the time to connect. CPDN, SETI, Einstein and some others always have work, are well maintained and the science is really cool. Choose wisely!
Sorceress... thank you thank you thank you!
Ultimately, yes, that's the best advice. If you run a PPC Mac like me, then yes, you have to see what projects will even run. (Some small ones are Windows or Linux only.) The level of support and development are critical. Check the tone of the users, and the tone of the admins. For that matter, see if anyone is posting that looks like they're actually part of the project. To me, I want to know what the science is, what's being done with it, and who will "own" or work with it. (Also note that BOINC'ing, and Folding and GRIDing, are international. Do *not* fall into the "Ugly American" stereotype of assuming poor grammar shows a lack of intelligence. I've seen kids (mentally, probably physically) do that, and it makes me weep. If communicating to an international crowd, be careful no to "dumb it down", as that can come across, but use short sentences, be concise and focused, stay on topic, and avoid metaphors or exaggerations. Also, the other party may be having someone translate back and forth, so be generous with time when expecting a response.)
Plan to try a couple work units, a sometimes a particular chipset / OS / anti-malware / firewall / Mercury going into the wrong astrological house while retrograde will make things just not work. I crunch for Superlink. It works fine on my Linux box(es, now one), and my G5 Mac, but in 8 months I've only had 3 or 4 work units work on my Windows XP / AMD Athlon PC. The last time I attempted to run it, on that PC, I had a ~50% failure rate, as opposed to 100%. Don't know why, but it runs on other boxes. Anyway, if you're unsure, link one box. If the results are inconsistent, but it sounds like the project is developing, leave just one box attached, and set the time slice to 50 or 25, and check results on the web site weekly or so.
Actually, that's another good point. When you start a project, check your profile on the web site every few days at first, then weekly for a bit. Check your pending credit and results per computer. Make sure your results are getting results.
The amount and rate of work is important if you run multiple projects. If all your projects are intermittent, you'll have a feast or famine scenario.
Well, I'm prattling on here... Enjoy your years of BOINC'ing! As Sorceress said, do some research, and ultimately choose what interests and suits you.