At Bikes Not Bombs, we use CiviCRM and are very happy with it. Overall I'd say it's a terrific solution for an organization with a little bit of tech resources, but probably not so well suited for smaller groups and collectives that don't have easy access to IT skill/infrastructure. Here are some more detailed thoughts:

• Good
• Free and open source
  
• Integrates with Drupal, Joomla!, and WordPress (we're using Drupal)
  
• Tons of great features! We use it to track all of our individual donors, grant applications, volunteers, special events, program participants, newsletter subscribers, and memberships. It's great at interfacing directly without supporters though our website, allowing them to make donations, RSVP or buy tickets to events, set up their own personal fundraising pages, and sign up for our mailings -- all saving us a lot of data entry work!
  
• Vibrant community of other users and developers
• Upgrades released often with many new features always emerging
  
• Challenging
  
• You have to install it on a web server and keep it up to date which requires someone with a bit of advanced computer skills. You'll also need to create a good system for backing up your database.
  
• The web server you use will have to have some substantial resources. We use a Linode VPS with the 1024 plan for $40 per month. You can run it on a shared hosting plan for way cheaper, but I wouldn't recommend it because it will be too slow.
  
• Sending bulk email from CiviCRM requires that you either maintain your own mail server (not recommended unless you really know what you're doing), or pay additional costs for an external email service like CiviSMTP, SendGrid, or Constant Contact.
  

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Sean Madsen
Bikes Not Bombs
Information Technology Manager
sean@bikesnotbombs.org
617-522-0222 x111
284 Amory Street, Jamaica Plain, MA 02130