⟨12Gratak-⟩ The MJ prinicipia execution is actually surprisingly well implemented: 
1. If it starts late (ullage problems, didn't manage direction in time or simply pressed too late) it keeps going after the maneuver ends and keeps pointing (mostly, see issue 1) in the right direction
2. It can actually handle in-maneuver staging decently. You have to stage manually and it will be off more then otherwise, but it does the right total burn time (increased if the next stage has lower twr). I actually use this often when my departure stage is a bit too weak since there is no good way of setting up two nodes for this in principia ahead of time and you'd be wasting precious time near earth if you do it in between the burns. You do...
... waste one ignition this way though (as the correction burn will usually require the main engine again in this case)
3. When none of those problems appear and you set the tolerance in accordance to your twr, it is very precise
The small issues I have are the following:
1. It stays in "fixed innertia" in case 1 once the maneuver is over for principia
2. It starts ullage late (lead time doesn't seem to do anything)
3. The auto time warp is badly implemented. It assumed setting SAS will fix the direction and is thus basically unusable. It will fix the direction and warp until 10 seconds before the maneuver. It's then often way off and doesn't manage to get the direction correct in time