The nice thing about Frame Relay is that it's cheaper than a DS1/T1 for similar speed. Since Frame Relay works over existing digital circuits, rather than requiring a new one to be installed, an ISP has a considerably lower cost with regard to line charges.
Frame Relay is done by many ISPs, not just AT&T. The data is assembled into variable-sized packets, and routed to its destination through possibly many "hops" through various existing digital circuits, owned by the local telco. This makes Frame Relay's bandwidth dependent upon the available bandwidth of the circuits over which it travels. Frame Relay can also induce delays in data transmission, caused both by the packet assembly process, and by any bandwidth restrictions in its path.
These caveats taken into account, Frame Relay is still an excellent choice for connectivity.