We recommend and use CiviCRM with most of our clients, and have since 2005. It's got a fantastic collection of functionality that fits the needs of non-profit organization communications, and the CiviCRM community of developers and users is growing, broad, vibrant and responsive.
The best part? When I describe to potential new converts how all of their constituent relations (donations, membership, mass emails, etc.) can be managed with a single integrated, configurable tool, I can hear an incredible yearning at the other end of the phone.
Freeform Solutions uses CiviCRM for our internal CRM. We are also a NFP IT support organization and we implement CiviCRM for NFP organizations we work for because we find that CiviCRM is the best open source CRM out there.
The CiviCRM community has been a tremendous resource for new ideas and helping us solve problems. We are excited to contribute customizations EFF makes back to core and support new features such as batch entry for offline donations or multiple payment processors on one donation form.