We sell Trixbox as well (among other solutions). It is very feature rich and extremely extensible through the use of AGI scripting. Its very easy to tie IVRs in with databases and other applications depending on business need -- for example, do stock lookups or ticket status updates.

We've got quite a few installed with no real issues related to the system itself, always vendor issues or network issues. We've settled on Dell servers, Dell PoE switches, Fortigate firewalls, and Aastra or Polycom phones with very good luck. Redoing the entire network to support a solid VoIP implementation isn't cheap, but the customer is also (usually) getting a much-upgraded data network over what they had.

Yep, you're not going to get SLA on Trixbox (reliably) at this point. There are some people working towards that goal and it sort of works, but I wouldn't call it mainstream-ready yet. If that's a deal killer for the client -- there's other options that would be a better fit.

As long as you use quality hardware, set up the data network correctly, and follow their best practices, its rock solid. We only do the Pro version so that's what I can speak to -- it does use a stripped down / hardened CentOS and Asterisk core that's not bleeding edge, but stable.

Like any other VoIP system -- it has its place. Use it for what its designed for and I think you'll be very pleased. Try to fit it in somewhere where it isn't a fit, and you'll have issues.