The bottom line is that it lacks a lot of the functions that have been available in Sonar for a long time. As far as bugs go, the current updates to lower the latency of the audio engine when used in conjunction with their Quantum interfaces has left a few hiccups. Copy/paste and rendering when using automation is hit or miss. Also, the program skips reading random midi notes since the last update. Studio One has great development though, and I'm sure they'll get it sussed.
To expand the MIDI functionality of Studio One, check out the Studio One X addon.*
*(I originally posted the link to Studio One X, but it looks like the site has been gone for days)
If you're seriously into MIDI, you're much better off with Cubase. The timing and functionality has been rock solid here.