Matt,
I have a full T-1 ant our main office running an H.323 Gatekeeper (GnuGateKeeper to be exact).
Branch offices are connected via DSL, Cable, and Wireless Internet. Speeds range between 1.5/384 to 768/256. Ping rates from main site to branch locations are 120ms max.
Softphones on users desktop is SJPhone by SJLabs.
Quality is very good. It's tough to say how many concurrent calls I can handle. I have about 70 users and haven't had problems, but it's tough getting folks to use the VOIP becuase the regular phone is what they're used to using.
Good luck.