Four headphone channels, which is pretty much standard, but along with per-channel bass and treble controls, it also provides an AUX IN per channel, that lets you -- in my case run an OUT from the preamp into this IN to put "more me" in the headphone mix. That is, you can balance the signal that's coming out of the mic pre, with the backing track AT THE HARDWARE without adjusting or having to effect the software levels that are being recorded.
You get a lot of control for the price.
The one area that may be problematic and worth mentioning, is that this headphone amp might be a bit underpowered for certain scenarios. This amp is rated at 140W spread over 4 channels, i.e. each channel will only receive 35 watts. That enough wattage for my recording needs currently, but if you're trying to hear your backing tracks over, say live drums, it might not be enough power in some environments.
There are plenty of good headphone amps available, but this one seemed to do a lot for $169. There are other headphone amps that provide more power per channel, but seemed to have less flexibility. Of course, you can spend a lot more if you need to as well.