A true T1 is only 1.5mbit, as it's 24 64kbit streams tied together. A T1C, or more commonly called a DS1C does ~3mbit, and is likely what they have.

T1 line benefits are pretty much what you said. People get them for a guaranteed speed that they could sit and burst on all day with no impact. They are also great for doing dedicated voice communications between places, much like ISDN. The speed is both ways as well, so you get a solid 1.5mbit up and down. Usually T1s can also be given a lot of IPs very easily, as T1 providers tend to have enough to go around.

Switching to FIOS might be worth it, but make sure that whatever the agreement is, there aren't any hidden usage limits, or other issues that may make it unsuited for business use.