We currently are using Trixbox and while its fairly reliable I'm thinking of moving to Elastix. We are running Trixbox 2.6 and I have never been able to get some features to work such as voicemail to email and I also feel that its a bit cobbled together around FreePBX. We are going to require fax to email support and I'm not confident that Trixbox can do this reliably, and if it can that it will not be easy to setup.
As a replacement I'm looking at Elastix. I see an Active Directory integration module is in the pipe (which is a big thumbs up). Everything I hear is that its better than trixbox and also has better fax support.
We are on DVX with 11 DDIs, 2 trunks and 4 simultaneous calls.