Seconded, sort of. For bigger installations, I stay away from trixbox and just use asterisk and sometimes freepbx on top. To get up and running fast, trixbox is certainly one way to do it. Also look at PBX In a Flash as a potentially better alternative.
Our company is similar to yours in size - 15 people. Call volume is a little higher - 162 calls today according to the logs, and we have very few problems. We've had boxes die in the past so backups are key, but don't cheap out on the hardware even though you can. We use both TDM cards and SIP providers and both work well.
I should say this is only viable if someone is going to take the time to learn the basics of the system, install it, and support it. But you do not need a huge amount of experience to get a full featured system up and running.
We have all the features we need or want, plus many options on physical phones. Mostly we use Aastra but have used and tested Polycom, Grandstream, Cisco, and a few others.
The best part is the freedom - add as many extensions, mailboxes, etc. to the system as you need. Have it email your messages, page you, forward to a custom application. And we are opening a branch office so we will be trunking between them to reduce costs on long distance, all basically out of the box.