It depends on the release. I've been a member of Connect since for the last 4 years or so.
The major benefit is to beta test software and provide feedback to Microsoft. If your feedback is of interest, you'll be contacted to provide more information - and that may result in an improvement of the product that you're testing.
For example, I had a lot of BSOD's due to my network card while beta testing Vista. I worked with Microsoft to identify the exact cause for them, and when they fixed it they had me test the next version of the beta to make sure the fix worked. Then, as a reward, I received a free copy of Vista when it was finally released (not all beta testers get this, only those who've provided assistance).
Once accepted into Connect, there are "open" betas for you to test, and there are beta's that require an application, and maybe even filling out some questionaires.
I've participated in about 12 testing programs since enrolling in it, with tracking enabled for 4 other programs. I've still got a couple of invites that are still open - but I just don't have the time to keep up with all that's offered.
While it doesn't happen all that often, how is Microsoft going to contact you if they need your input, or need to send you something (like I got my copy of Vista)?
