I'm on the other side, have been doing stand-up for a while but want (need?) to get into improv instead, for similar reasons. In Chicago it's fairly welcoming, though. My perception of Boston is a bit more 'macho' and dense - for every three comedians aping Louis CK's worst traits in Chicago, I imagine that in Boston there are dozens.
That said, my biggest issue with open mic's is the limited time. I know it's necessary to start this way, but I always wish I just had a minute more to prattle on.
Yeah, I'm planning on moving to Chicago or New York within a year, just for comedy reasons. I like Boston, but it's not gonna happen here, and now is the time to find out if it's going to happen anywhere! I also know a few people at UCB/Second City and really wanna get in on that, because they are such great places to do comedy.