I’m on 5.1 (not the hotfix)
Use it every day. It does crash about once a day, sometimes more. Probably a rogue plugin… can’t figure it out so I expect it as the norm.
If you are coming from v3 or v4, save times will be slower I think. What else… can’t think of anything. Been on 5.1 for a long time.
Well, Nuendo and Cubase (all versions) are not crashing in regular usage over here - usually Nuendo 5 or Cubase 5 is running about 70 hours a week. This might be indeed a plugin.
I sometimes (not often) have crashes (all versions) when I use for example Autotune for offline processing or sometimes when closing a project. When just recording and editing audio I can work for weeks without a hickup.
In N5.1 there are no big showstoppers, beside the bug “which can not be fixed without reprogramming the audioengine” with the disconnected VSTi metronome (you have to set it up again when you open a song).
But I did not spend much time in N5.1. I was in Cubase 5.5 for a long time beside Nuendo 4.3 - upgraded to N5 just because of N5.5. I installed 5.1 AFTER I checked out 5.5.
Well, I can’t speak for N5 because I am not using it - but between N4.3 and Cubase 5 (and 5.5) is a big step - at least for me. It is close enough so everything is familiar and compatible (I am able to excange projects between C5 and N4.3) but a lot of details are better. I can’t recall in the moment… Midistuff - HalionOne for example, lots of smaller improvements, editing - that Melodyne stile stuff for example in the sampleeditor. I realy can’t recall in the moment but immediately I was not able to use 4.3 anymore because I became used to the 5 stuff… (not talking about GUI and things, you know 
Well, as I assume N5.1 to be basically a better Cubase 5.5 I would highly recommend checking it out!
I am currently that much into work that I don’t feel the need to swap to N5.1 when C5.5 does it quite good
Hehe, sounds almost similar to my situation
I want a Nuendo with the N5.1 GUI tweaked with the show options of Folder tracks, the old lane concept, Multitrack/group editing of N5.5/C6 + fixes for the Event on top issue and some other small things.
This would be a perfect Nuendo.