A. T1 was the standard for a long time and is still prevalent today, however, there are more options and better values than T1.
A dedicated T1 line with 1.5 x 1.5 (mb/second, upload x download) will cost approximately $300+/month. However, cable providers (TimeWarner in NYC) are now installing 35x5 (mb/s, up/down) service for $150+/month. While cable service will fluctuate based on time of day and number of users (much like a shared circuit), it is still a better value than T1 service.
The best choice, however, is a fiber connection (Verizon FIOS in NYC, where available or Cogent) which has become relatively inexpensive at approximately $300/momth for 50x25 (mb/s, u/d) service.
Many companies today are employing cable or fiber service for data usage while emplying VOIP telephony over a dedicated line, either a T1 pair (3x3 mb/s, u/d for $600/month) or EOC (ethernet over copper, 5x5 mb/s, u/d for $500/month), for built-in redundancy. This way if outages occur, the use can send data over the phone lines (or vice versa) by employing an inexpensive router ($400).
In short, while T1 still exists, there are multiple connectivity options to choose from. The days of the fluctuating, $100/month DSL line, however, are over!