What Cris and markat said is completely correct (I actually would go with the new devices) but if that is not an option for you - either because they were already purchased or you have some spare ATAs you want to use- its important to keep in mind the following:

ATAs do not support modems nor any V.33 based device,like SuperG3 faxes if you want to use a 33.6 kbps fax with the ATA it most likely won't work, even if you set the speed to 9.6kbps its always a v.33 modem trying to train up at a lower speed.

Its important that QoS is implemented and that you keep your ATAs away from any source of multicast traffic or heavy IP traffic, since they do not respond very well (CPU goes high and they reboot themselves) due to a hardware problem they share with some old IP phones like the 7905.

If at all possible make sure that teh ATA is on a G711u region, there are 2 reasons for that, if you have not configured the voice gateway for faxing faxes might work without that configuration because G711 does not compress the fax tones. ATAs only support one port with G729 even though you have both ports on a G729 region, the second port will always use G711.

If you are going to use only one port of the ATA make sure you disable the registration of the other port, otherwise just like an IP Phone that can't get registered the ATA will reboot itself and you will get reports of dropped calls.

The second port can be disabled on the ATA GUI (http://ATAIPaddress)

look for SCCP Parameters and then look for SID1, change the dot with a 0 to disable the second port.

I have not had the chance to play with the new vgs to see if there are any surprises like the ones I mentioned but only the future will say :), still the ATA works great if you know its limitations.

Hope this helps!

- Manny