I just got a GSP1101 and so far, things are running great with Mark V in a 4CM setup. I'm not using amp/cab simulators (not with MKV at least), but everything else seems to work great. For now, I'm mainly using reverb, delay, compressor, gate, and chorus, but I'd like to experiment with throwing an OD in there every once in a while. The OD gets tricky because it tends to color the MKV tone, so I have to be careful with that.
I have the GSP loaded with c63 beta firmware, and it opens up lots of additional options. For example, now you can choose where the Gate sits in the chain (before or after MKV preamp in 4CM), while STILL being triggered early in the chain (before the preamp). I would recommend the beta over official DigiTech software.