I just looked it up. Looks with a MEAP subscription to a book, the reader would get to read the chapters as and when written by the author.
And once the whole book is completed, the subscriber receives the final copy.
What's interesting is to hear about the forums and how the reader could discuss with or post feedback to the author during the process.
Yay ! Being more involved than just "reading" - I'd like that
Yes, the Manning Early Access Program is a nice way to get an early look at the titles they are working on. I own a bunch of them, too. I wanted to reply to the poster who mentioned re-buyint a certain title. I hope you're aware that you can go to beta.manning.com and log in, and there you will see all of the titles you own. (You can also download latest versions or additional formats or whatever.)
When you get in on these MEAPs, you find out just how much improvement is made from the first published draft of a chapter to the finished product.