We've used Cabin Charter Sailing a couple of times when we couldn't get a full boat together ourselves. http://cabinchartersailing.com/
We were concerned the first time about people we didn't know, but figured most people willing to look into a cabin only would be fairly laid back and easy going. (and if you weren't thrilled with someone, you could always wander off on your own when you landed each day).
Maybe we've just been very lucky, but we have enjoyed meeting a bunch of different people and still sail with a couple we meet on our first cabin only trip a few years ago.