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.
CiviCRM has one of the most active and friendliest communities I have come across. From initial tentative forum posts I was encouraged into engaging more actively through IRC and directly with other groups & individuals and am now happy to count many community members as friends. I recently found an article on the web that said if you post a question about CiviCRM anywhere on the web Lobo will post an answer within a few hours. It often feels like that is true.
One of the most valuable way in which the community supports me is by allowing me to bounce my ideas around and often someone is able to suggest an approach which is better than mine.
We feel there are too many obstacles facing not-for-profits (NFPs) considering commercial CRM offerings, including many of those that are charity oriented. From licensing models which restrict the fluid expansion of an organisation's user base (why should you be punished with higher costs for being successful?), to support from commercial companies being inherently tied to one supplier; a NFP would benefit from the option to 'shop around' for those most appropriate, e.g. based on: proximity and availability on-site, cost, experience, value added services... They also often lack the capacity for charity relevant workflows, necessitating either customisations, complicated and inefficient workarounds or an en-masse call for new functionality, as individual charities do not appear to carry the weight required to influence subtle NFP-only changes to market leading software, without large expense.
On the flip side, CiviCRM is completely free and open-source, carrying with it a friendly, hard-working and enthusiastic community of developers and implementers, constantly listening to the users' needs and sculpting future releases to the requirements of NFP organisations. This is exciting!
The CiviCRM community is a great place for support, to exchange ideas and to contribute back. Working with other developers or users has often allowed me to pool our resources together and lower our costs, while ensuring better quality since there were more people using it.