We use Sage CRM here and though it is customizable to work with GTD (sort of) I find myself migrating back to simply using Outlook to manage tasks and projects and use CRM only as a Database and simple marketing tool.

With Sage you can intergrate with Outlook, but it is rather cumbersome to do so. To file an email against a customer's file is at least a 2 or 3 step process, which is completely counterintuitive to the GTD mentality (and is way too easy to simply not do).

On top of that, you can use SAGE CRM to send email, but not receive them.
And who wants that? It's like having a car that turns right but not left!

I have friends who are in the business of being computer nerds and they swear by Microsoft CRM only! (perhaps because they're MS Platinum dealers?).

I have played with Microsoft CRM a little bit, and it seems to be a great product.