Well the only problem I've had with Reaper's MIDI is the infernal loop bar every region has by default, as well as clicking and dragging regions sometimes making mystery notes play when they're not there and stuff, but it's nothing beyond a minor annoyance for me (though all I use it for is drum and keyboard programming). I wouldn't say it's by any means a deal-breaker! And yeah, for mixing/editing of a lot of tracks, there's no reason not to up the buffer size, since latency doesn't matter!