CiviCRM Gets the Job Done
I established CiviCRM for our organization almost 8 years ago, and I'm very happy we made the change to it. It's come a long way in those 8 years, but the best thing I can say about it is that it enables staff to do our jobs. We can easily retrieve records, make changes, run reports, and all of the other back-office activities that are required for association professionals. It also does all of the things we need an AMS to do, including:
Membership management
Contact management
Email/newsletter management
Event management
There are definitely things that I wish it did better, but in general we're able to easily find work-arounds for most limitations. The very best part is that there are no licenses, it integrates pretty seamlessly with our CMS, and updates are not too difficult. You will need a developer either on staff or on retainer/contract to help manage initial implementation and upgrades, however day-to-day work is easily managed by staff with fairly basic training.