Well I can't speak for BOINC Synergy... but as far as the benefits of joining a team...
1) Support: When you have issues with your OS, conflicts between BOINC and other programs, or just your computer in general, chances are someone on your team (especially if you are in a larger team) has seen it before and can offer helpful advice.
2) Friendships: Many of the larger teams have active forums, where you can hang out and chat with people that you know right off share at a minimum one interest with you.
3) Challenges: Team efforts are fun, many teams participate in challenges such as the SIMAP scramble, the PrimeGrid Challenges, or other challenges that are run from time to time on various projects. Most people can't compete with those that run huge farms and have giant budgets for BOINC, but by combining your efforts with your team, you can still make strong showings in challenges.
I have been looking around at the teams that are available, and I'm interested in what are the pros / cons of joining a team. If there is anyone out there in team
BOINC Synergy, please answer since you are the team I am most interested in.

Best of luck with whatever team you decide on, and remember BOINCstats is always looking for new, active members that are interested in participating in team efforts