The cost savings can be significant. I just brought up two locations for my employer with 45 extensions at one location and 33 extensions at the other. They each have a single PRI line and are connected with a point-to-point T1 line that carries both voice and data traffic between the locations. Total cost for the phones (73 Snom 320s and 5 Snom 360s with sidecars), two asterisk servers with Sangoma A102 cards, two Linux based routers with Sangoma A102 cards, 96 ports of switched Ethernet with POE, and other misc. networking support gear totalled close to $45K US.
As a comparison, the cheapest bid I got for just the location with 45 phones was slightly more than $50K. That was from a 3COM dealer and wouldn’t have included a lot of the network odds and ends.
The downside to doing it yourself (for me anyway) is as follows:
Training - Have the entire system operational and create the necessary guides and cheat sheets before EVER letting someone touch a phone.
Workload - I have other things to do so supporting this system initially ate up a lot of my time.
Best Practices - I was talked into changing some of my default timeouts to shorter periods which caused problems while people were still learning the system. They ended up needing more time to complete transfers, etc. than they thought.
Feature Creep - Can it do ____? Because it is capable of so much and there is no additional cost in enabling a feature (as opposed to something like 3COM or Cisco) management is more likely to ask for features. Nail down the feature set and hold to it.