I've had the Impact Twin for a little over 3 months now and it's one of the better investments I'm made for my writer's studio. This unit replaced an aging Delta 1010 that sounded very good as well. The difference between the two unit's sound is noticeable. While the Delta 1010 offer a rich full sound, it also lacks in clarity compared to the Impact Twin, which also has a rich full sound.
The preamps on the Impact Twin are above average for interfaces. The DSP monitoring with reverb is great. I found myself using the included TC VST reverb plugin on some of the tracks after they were recorded. Eventhough I have other good reverb plugins to choose from and normally would use them. However, the TC reverb is rich, full and clear.
Also, if you have an outboard preamp, the Impact Twin's rear inputs allow for the signal to bypass the internal preamps which is a feature I used as a decision maker when shopping interfaces.
Headphone amp has plenty of power which is a plus. The included TC Near software is elegant and useful (much better than the m-audio software I used for years).
The nearly zero latency monitoring surprised me. Of course when recording live playing VSTi plugins I can set the Impact Twin to 64 sample size and it handles that task very well. But often I move the buffer size up to 512 for mixing or using a program like Melodyne. Using the mix knob on the Impact Twin, I can still record vox, gtrs, etc., at near zero latency with the larger buffer setting. Again, the dsp effects come in to play here. They sound great.
Now that I own and use a Impact Twin, I don't see wanting any other interface to record with. As much as I like new stuff, I wouldn't trade this unit for another right now. Including the next step up higher priced interfaces I've heard. The TC Impact Twin is that good.