For me the charm of the Nikon Rf's is the fact that until 15 years ago I never owned one. I had used them - occasional "trading" with other shooters in the 60's and 70's - so there was a great deal of the mystique surrounding them. I started with M's in the late 50's - and still use them - but there is something about an S2 and a 50f1.4 or a SP with the 28/35 that is appealing.
The lenses are good, in many ways better than Leica's offerings at the time and they are robust. I have had very few Nikon Rf's that has given me trouble. I had a S4 that capped the shutter, a S2 with a weak rangefinder spot and a S3 with iffy shutterspeeds. I just like them because they are different.
It is easier to pin-point focus with a Nikon. The longer throw makes it less likely to "overshoot" the target.
OK, the "toothy" focussing wheel is not the brightest of designs and changing lenses is a bit of a hassle. Easier to keep several bodies and just leave the lenses on them!
Some of the lenses are wonderfully vintage, the 25f4 has a great vignetting, the 21f4 (F-mount with adapter) is sharp - but you certainly never have to edgeburn a print!
The 35f1.8 (both the old one and the new one) are spectacular performer- sharp and nicely moderated contrast. The 50f1.4's (old and new) will give Leica's f1.4's a run for the money! - And the 105f2.5 is still one of the best "short" tele's for a Rf.
As for cost - stay away from the collectibles, the original Rf mount 21, the 50f1.1 (not that great a lens anyway) and the 85f1.5 is slightly better than the Summarex from Leica = but that doesn't say much!
I usually shoot M's for a couple of months, then switch and shoot Nikon Rf's for a while as i cant really shoot them side-by-side as the "wrong way" focussing is difficult to cope with.
I you have to ask - i suggest you splurge on a S2 and a 50f1.4 ( will cost you less than a Leica M and a lens anyway) and try it out.
Oh, another benefit - Nikon Historical Society has the best "conventions" - in 2010 it is in Bruges/Belgium - great beer and usually very good programs. I have been to about 5 of these meets already and I am going to the one in Belgium too.