Yeah, man. I've been saying it since I figured it out... speedcomping in X3 is the shizwhizzle. The CakeTV entry on speedcomping has pretty much all the info to become a speedcomping master (for those who don't have access to the Groove3 X3 vids which unfortunately I do not) as far as I can tell but it does move very quickly so lots of pausing and note taking is advisable. Because I'm away from the DAW so much lately I still keep my little comping cheatsheet on my desk. After a 20 second review I'm editing and splicing like a champ. So a little more complex than the old methods I'd say at the onset and it takes a bit of remembering (key commands, hotspots, etc...) but once you get into the flow it saves HOURS of painstaking bullpuckery (for me anyway). Also I never EVER liked the way auto crossfades sounded in the past within any of the (admittedly few) DAWs I've worked with so I'd spend forever cropping and creating fades. For some reason the autofades with X3 are perfect almost every darned time. Actually with X2 I couldn't even get any take overlaps to sound good (something was degrading the signal) so I actually created new tracks for any overlapping parts then bounced down which was time consuming and annoying. Not sure what the what that was all about and generally I had some major annoyances editing in X2 that have all been corrected in X3 and then some.
As far as using speedcomp with MIDI? I use it there too and it works very well for how I do things. I like to play as much of my MIDI drums live on my padKontrol as I can because it is a heck of a lot easier to get my weirdo beats down that way instead of trying to do the hunt and peck in PRV. The pK sure isn't anything like a real drum kit so there are a lot of dropped/doubled triggers and/or human error. I also like to play around with different ideas and fills so being able to tap out a bunch of versions and then splicing together what sounds cool (and then doing any correction work or fancying up the parts afterward in PRV) is very liberating while writing. I haven't done much keyboard work yet (for multiple reasons) but if I get to live key input I'm sure I'd use speedcomping in the same way. The one problem though is apparently every time you make a split on a MIDI track it creates two full versions of the same file whereas with audio it will draw from the original file. I understand why this is (kind of) but the end result is it ends up turning say 5 takes into 25 if you split it 5 times. I do WAY more takes than 5 and WAY more splits than 5 so it gets very resource intensive and has caused problems. Upping the hard disk read/write buffers fixes it but I figure there must be some smart guy thing the Bakers could do to the software to avoid this.
Anyway... blah blah blah.
tl;dr... Speedcomping rules... even for MIDI... hope all is well in sharkeland and all the lands of my fellow Cakesters.
Cheers.