I take advantage of their legal services plan, and have also been using a number of the regular services they offer that are encompassed within the normal membership benefits (i.e. medical staff help, outreach programs, etc.). I do not use them for my aviation insurance only because at the time I was getting my renters policy they didn't have a category that fit me at the time. I've heard that has since changed, and their rates are competitve. I use AVEMCO for my renter's policy. The legal services plan is probably one of the best "bangs for the buck" around. I hope never to have to use them, but I know my $12.00 per year is cheap for that level of peace of mind.
I already have life insurance coverage outside of AOPA, so I don't take advantage of that with them. I do have the AOPA Visa card, which is nice. You can charge your plane rental, fuel, and purchases of other items with it, and you get 5% back on those purchases (provided the vendor is a participant... which a lot are) up to $250.00 per year. It helps defray the costs of flying even more as a result. Sporty's Pilot Shop gives you the 5% with your AOPA number at checkout too, which is another plus.
Other things I take advantage of are the ePilot magazines, which are emailings of articles for both the AOPA Flight Training, and AOPA Pilot magazines, which are great to have. Experienced, and student pilots alike can benefit from the AOPA Flight Training magazine. My daughter is a prospective pilot with 12 hours in her logbook, so I got that for her, and found that I read it as much as she does!

It's nice to have it in electronic form too.
I also use the PATH to Aviation area to get materials that I hand out when I do my volunteer work at the St. Louis Science Center in the Mission Control: Take the Control's exhibit. Brochures, pamphlets, plastic "Future Pilot" wings, puzzle books, etc. for all ages are created by AOPA to further aviation interest and knowledge.
I also am registered with them as a Pilot Mentor (a free program) that matches pilots to propective pilots to act as mentors through their flight training and help the student through the phases of their instruction. This is not something that you have to be a CFI to participate in, as you are not giving instruction, just acting as a guide. AOPA provides the information and resources for the Pilot Mentor. As a mentor, I can sponsor someone through the program, and they get six months of membership and AOPA Flight Training magazine free for those six months. After completion of the Private Pilot certificate, the student is awarded a gift certificate to Sporty's Pilot Shop (currently a $50.00 value) on behalf of AOPA and Sporty's.
As I noted above, I take advantage of the AOPA Medical staff and Turbo Medical Application for help with my going through the hurdles for my Class III (I'm on a special issuance), and they have helped me tremendously.
Let's see... what else... oh... I like their photo gallery... great for research if you don't know what a given plane looks like

It also has some wonderful images in it that make great wallpaper, etc.
