I use a Focusrite Scarlett 2i2 with Ardour and Ubuntu Studio. An aside - the only bummer about the Focusrite S 2i2 that I have at least is that it HAS to be plugged into a PC USB port in order to power it, even if, for example you want to just use the mic preamps live. Wonder how Focusrite preamps would compare to the Helix mic preamp effect block? I've never considered using the Helix as a USB audio interface but I do use it to make my coffee. Is there anything it CAN'T do..
I use a Zoom R24 w/o a computer for basic recording. 8 inputs, no noticeable latency, no system configurations, and two condenser mics that I can easily use to record live drums or multi track simple acoustic guitar and vocal parts. A little primitive and a bit awkward to use when it comes to adding effects, but you can always move the WAV to a PC and use Audacity or whatever on it.