I've been a member of WPMU DEV for over 6 months and have to say it is pretty awesome.
Not only do you get access to every plugin you could possibly need (not to mention incredibly powerful ones like a Membership, Affiliate, eCommerce plugins and everything you'd need to create you own income generating networks) but they also provide outstanding customer support.
I'm a mid level website designer and have many questions that pop up from simple to complex. I have never been disappointed with their ability to answer those technical questions and their response time is amazingly fast.
It's literally like having a team to help.
I highly recommend them to any WordPress developer looking to become more profitable and efficient.