Anyone use VoIP phones? What do you think of it and the quality of service?

We are looking at getting VoIP, who should I ask about it?
-Should I get it through my current telephone company, my ISP or someone else?
-What factors, or hidden things are there to concider?
-Should I look at getting a separate internet line for the phones?
-Will it save us money? What kind of costs are involved in the up front setup?

This is for a small business with 9 phone lines, 2 fax machines, and a small network (16 computers, 1 server)