korhanerel any device that is supported by ALSA on Linux should work. I succesfully tested the Focusrite e Scarlett 8i6 (midi only), Native Instruments Kontrol S 49, an M-Audio Axiom AIR mini 32 and a couple more with mini-keys (don't remember which at the moment, sorry). Any class-compliant device should just work.
nuromantix We have not done anything like that so far, but keep in mind that Midi serial speed is pretty slow, so a three-byte midi message takes just below 1ms to transmit/receive (serial rate 31250bps and each byte has to have a start and stop bit) . On the other hand, if you really want to, you should be able to reliably sample it switching the analog in sampling frequency to 88.2kHz (i.e.: setting the number of analog channels to 2). I think you could probably get away with sampling it with the digital ins at 44.1kHz, but have not tried that. Not sure how you would handle MIDI serial out, instead: the clock frequency of 31250bps is not achievable (well, you could slow down the audio clock if you really have to), so it will depend how tolerant the receiving device will be, but 44.1/31.25 seems like a big stretch ...
Also do not forget you need an appropriate optocoupler as per the MIDI specs.