SuiteCRM

It's easy to use and to customize. We've adapted to some of our internal processes and at the same time we adapted some processes to the SuiteCRM way.
Has a a lot of features and some new one are released frequently enough.
As an Open Source project is easy to integrate with other platform.
Ii love the "no license" approach because I can't think of a CRM solution not to be used by all the peopple in a company.

Community support is not always very quick, but is part of the game.
Some integrations (Google calendar) are still not completely working well.

Without going feature by feature I think it's fair to say SuiteCRM is a drop-in replacement for popular open source and SaaS CRM (customer relationship management) applications. I use the open source community version on an in-house server for internal operations. I've also set it up for a young non-profit organization to manage constituent, board and funder relationships. The software is popular enough that it is included in the group of softwares that can be automatically installed on a hosting account. I'm happy that the developers update it on a regular schedule as well as pay close attention to bugs that crop up.

SuiteCRM is not as polished or aesthetically pleasing as some of the others in this space. Feature for feature it's a good alternative but many of the features either lack intuitiveness or depth or ease of use. They feel rushed and would do better with maturation.

It's Free
Able To Host It On Your Own Server Instead of In The Cloud
It's Simple To Use
Ability To Create Your Own Modules
Lots Of Other Modules Are Available For Free or Paid In The Community
These Modules Allow You To Do Just About Anything With This CRM

Their Support Is Not Very Good - I Have Posted In Their Forums Seeking Support And Never Heard Back
They Don't Provide Phone Support For The Free Version
There Are A Lot Of Bugs In The Code
Sometimes Updates Fix One Bug and Break Another
You Have To Know Code To Fix All These Bugs
If You Don't Know What You're Doing You Can Mess Up The Code and Cripple The System

SuiteCRM allows us to add custom fields and change practically every screen and view, as well as data exports. This is critical as we refine our processes and change direction, we are able to reflect those changes in the CRM that we are using. Because of its Open Source foundation, we can change things pretty deeply, if we choose to.

There are sometimes slowdowns that make for a bit of "click and wait" syndrome. These have usually resolved themselves; in some cases we've had to have our sysadmin dig in and adjust some things to speed it back up.