We are using http://insight.ly - It gets the job done and integrates well with Google Apps. There are a few things lacking like an API and the reporting is pretty weak.

We are transitioning away to Zoho CRM. So far, Zoho CRM has been great but, can get costly if you have too many users.

My product integrates a CRM, so we dogfood. Works well enough, but sometimes you wonder if it blinds you to anything you're missing.

Everyone seems to like it much better than what we used to use, though.