I had the subscription as a first time cruiser. I also bought the book. I found the book really handy, as it laid out, in one place, the basics to start planning my cruise. The membership had links, but much of the information was elsewhere, including the book. I did use the 'reminder' to sign in, but then, I wasn't going to forget that awesome moment, 75 days before sailing. I also used to go to the forum, but found it really didn't have much throughput, so I eventually just came to the DIS instead.
All that said, every single time I go to WDW, I get the membership. I wouldn't do WDW without a customized plan through Touring Plans.