If you want one, get one. I did.
Almost everything here:
http://gallery.leica-users.org/v/freakscene/Russia/ was shot with the Nocti. Almost all my music work:
http://gallery.leica-users.org/v/freakscene/bands/ was done with it too. Everything here:
http://gallery.leica-users.org/v/freakscene/Noctilux/ is from the Nocti. Mine has paid for itself many times over and I use mine with my M8 a lot. Its main characteristic that I exploit is its flare resistance. Having f1 is a bonus for which I am grateful, but if it was an f1.2 or 1.4 lens I would still have bought it. Nothing else Leica has made comes close in this regard (including the 0.95 version, unfortunately).
Focus shift occurs when you stop down. This happens with this lens on film too, where the results are also apparent. You may need to get your M8 and Nocti calibrated to each other for optimum focus, but this also applies to film Ms (it did to my M7 0.85). Focus shift on the 0.95 is negligible, so if you are really worried about this, maybe that will guide you.
So if you want one the question is which one?
The f1 versions all have the same optical formula and the glass for the highest density elements was probably made in a single batch at the beginning of the first manufacturing run. It is flare resistant enough that a hood isn't really necessary except to protect that huge front element, but for that it is handy. For this reason I like the ones with removeable hoods or the f0.95 version best - the former have hoods that stay in place and the latter has a built-in hood that locks like the hood in the 50/1.4 asph. The f1 versions with the built-in hood have an incredibly irritating habit of the hood retracting when you want the hood to be extended.
Over time Leica probably updated their coating and QA procedures and tolerances. In this respect newer is probably better.
Older ones are more likely to have been used more and thus may be cheaper. The oldest ones have 58 mm filter threads which may be important if you already have a pile of 58 mmm filters, or if you have the Zeiss 18 mm, the new C-V 15 mm Heliar or any other 58 mm thread lenses. Alternately if you have the 60 mm version 75 Summilux, the newer 60 mm versions might be a better idea.
The only one I would recommend against is the first one, the f1.2, which is probably of more interest to collectors and although I haven't used one yet, I think if you want a slower lens the 50/1.4 asph is a better buy.
Good luck with your decision - I am sure Sherry will provide you with a great lens and don't forget to post lots of photos when you get it.
Marty