Ewan, interesting you should ask that, the owner of our small business asked me the same thing just the other day.
When we signed up at the end of last year, one idea we had was to be a member for 6 months or so, learn all there is to know so, then we could bail complete with all this new expertise and spend the $100 down the pub insead.
My answer was that Moz is well worth the investment. Since I took over the marketing role (I was sales manager before and still do that) the company was very traditional and conservative on the marketing side. Now we are hugely relying on inbound marketing and Moz is a big part of that for me. The phones are ringing, the emails are mailing so I know we are going in the right direction.
Sure there are a lot of free tools that can give much the same functionality as what we get here at Moz, but for me it isn't just the tools it is the complete package offered here.
When I get to my office in the morning (an hour before everyone else) after checking that our sites are still up, my first job is to read what's new at Moz. Being an SEO beginner, I learn something new here every day, from the two blogs, from the Q&A (lots of gems here!) and WBF and the webinars.
I know that a lot of this content is available to everyone regardless of being a paying customer, but to have just about everything I need to improve our sites and continue to get my head around inbound marketing in general here in one place is well worth the price as far as I'm concerned.
Maybe one day I really will know everything there is to know about SEO and inbound marketing and no longer need the Moz, I have a feeling that might take a while! 