I'd rent. As Rob said, some people got burned by AT&T by buying.
Somehow We made out really well by renting - we got them to replace the original (non-DOCSIS) modem when it started playing up (regular block-sync dropping), and then about 2 months later they rolled out a wide-scale modem upgrade with the same modem as our new one, and instructions to dispose of the 'old' one as we saw fit. This coincided with my in-laws signing up for cable and emailing me for advice on the same subject so they ended up with a free modem.
_________________________
Mk2a 60GB Blue. Serial 030102962
sig.mp3: File Format not Valid.