I've been using this as a temporary replacement amp with my HD800s and its pretty darn good. My old Earmax has reached the end of the road and is falling apart (its had a hard life), and the Focusrite bests it in most respects, although that may be because the Earmax isn't performing properly. The Earmax is ever-so-slightly smoother, but that's about it.
I can't compare the Focusrite to a functioning higher spec amp yet as I'm waiting for some loan units to arrive, but it has slam, pace and rhythm. It can get a little fatiguing over the long haul, but I can't really fault it at the price.
It has a single spdif input, which to my ears sounds better than usb only. It requires a PC to be switched on to work however so I can't power it from an ipod mains adaptor (which uses a usb cable). It also has an issue when using a 170i dock on the spdif input (with power taken from the PC) as it auto-mutes between songs, and occasionally at other times, causing every song to fade in slowly, as well as odd times in mid song. Initially I thought it was having difficulty driving the HD800s, but it has no such problems from the PC input (even using spdif) and it can go louder than I dare use it. Whether the muting issue is caused by amp or dock I can't say, although the dock drives my other dacs fine.
The software is vaguely amusing, but unnecessary from a headphone amp perspective. The box works just fine using the default drivers and they installed automatically when I plugged the usb cable into my Win7 64 machine. The VRM software is like a crude graphic equaliser with a bunch or presets*, and I can see it being very useful for mixing, but every speaker option mutilates the sound and the treble in particular. It's worth a laugh, but it's not worth extended listening. If they could use it to hone the perfect speaker set-up, with options such as Sonus Faber or Wilson, without strangling the treble then it might be worth trying again. One more thing, the drivers remember the settings, so if you set it to VRM mode and then uninstall the software you're stuck with the VRM tuned version until you reinstall the software and switch it off. Then you can uninstall it fine.
If all you want is a headphone amp though the hardware itself is excellent for the money. I'll do an A/B review with better amps (MF M1 dac and amp) in a week or so.
*I know there's more to it than that, but narrower sound-stage apart it sounds like a crude graphic equaliser.