My 0.02$:The cost seems pretty low (I read $25 per year), so even if it pays for a couple races I'm way ahead of the game. Hopefully, some people here can fill me in on other details I missed pro and con. I'm perfectly fine continuing to train and race on my own, I was just curious to see what clubs offer that I might otherwise be missing. There are some pretty competitive and experienced runners here (past and present) who can hopefully shed some light on this.
For me I didn't know jack shit about running so I joined the MRC. It was local, there were runners smarter than me so I was able to pick their brains, and I could go on group runs and do Sunday runs. If you poke through their message board, their Sunday runs are very organized and they (for the most part) get a large turnout with people with varying paces. I am still a member there but because I moved up North and many other reasons I haven't been able to get down there as much as I would like to.
GLRR isn't as big as it once was - I always hear stories of 100+ people showing up for track and for Sunday runs. These days, the Sunday runs aren't as big and the talent is pretty good for the most part. It's not uncommon to have two groups - a sub 7:00 pace and a 8:XX pace group but for the most part we don't have more than 6-8 people. GLRR also isn't as much of a social club as MRC - for the most part you can go sling 2$ beers at the KofC after a Tuesday night run and talk running with others. GLRR is really a long run/workout and then people scatter.
Some other local clubs that seem to be more wellrounded : CMS, MVS, SRR and the good old L Street. If you're looking for people to run with during marathon training - the biggest crowd is L street in Southie. There are always tons of people at varying paces and people from all clubs go down there and run from time to time.
For me, I look to clubs for camaraderie and breaking the monotony of long runs. As you meet people at your own pace - you tend to meet up for other runs. This is what happened to me and some of the Whirlaway guys in the winter - getting me some of the best workouts I've ever had.
If you want a contact for any local clubs let me know and I can get a name/email - overall the whole Greater Boston running community is a pretty close one with everyone knowing everyone ( except for the south shore - they don't count anyways )