IBPA is great and has a lot of really cool services and promotional options available for authors, but it can get extremely expensive very fast. I wouldn't sign up here unless you are actively trying to take your career to the next level and want help with purchasing expensive promotions that are shared with other authors.
They offer some great things. There are classes you can sign up for, shared promotions on places that normally cost thousands and thousands of dollars to be listed on, and a lot of other cool things that you couldn't find on your own. The people who run the website are great, and there is a strong sense of shared community throughout.
I have only done a handful of classes, but it seems they try very hard to offer unique and cheap content to their members that will actually help drive sales, which is something I think is great.
I have been a member for just under two years, and I still haven't managed to find my way through even half o the free content they give to authors, including articles, videos, and more. They also offer membership discounts for a lot of different services, but in general I haven't found these to be tremendously worthwhile. Most places already offer discounts outside of IBPA, and occasionally those discounts are better than the ones offered with the discount system.
Additional Things to Do
Probably the strongest part of working with IBPA is the group promotions. They set up monthly and annual advertising options where you can pay a little bit of money to have your book listed in a mass catalog sent to booksellers, libraries, and a multitude of other locations that would be willing to sell your title. I've participated in a few, and even when things went poorly (such as when they accidentally printed the wrong information on one of my ads) they were a pleasure to work with and quickly corrected the issue.
Should you sign up?
You can easily spend a lot of money with IBPA and get very minimal results, but you could also tap into an entirely new market that might create a long-term revenue stream. It is more of a service organization than a community, and in general paying the entry fee is only the foot in the door before you have to bust open your wallet for more money. But, if you are trying to push into the paperback market, this is a great place to start!
Check it out for yourself!