I've been using mouse/keyboard and playing almost exclusively in first-person so far.
this is pretty amazing. I'm already enjoying my 5-6 hours of GTAVPC more than the years on PS.
everything about this version makes the consoles look like ass. and the port itself is really clean and stable. 100x better than the GTAIV port at this same moment. so far there haven't been any crashes (haven't played online yet), glitches, or noticeable bugs. I'm running @ 1080 basically maxed out and it seems like the only framerate dips are happening more in cutscenes than during gameplay.
plus, the moment that somebody drops the first Multi-trainer, and I can start spawning cars out of thin air and editing the TIMECYC data, I'll probably quit my life and retire to my room until next Winter. I can't believe how sharp everything looks and how far the LOD details extend. and I'm not even playing on my TV like so many people I bet. as if the updated mechanics/visuals weren't great enough, the PC port actually works. this is so awesome.
everything seems to load faster, too.