I've got a few fast manual focus Nikon lenses and they are very nice to use, especially in terms of the view through them, but also for the creative options they present.
Of mine, possibly my fave is the 35mm f1.4. 35mm is a very versatile focal length, and still has a more usable amount of depth of field at wide apertures than a 50mm or 85mm. Really amazing for low-light situations too - can easily take hand-held shots at 1/30 sec and sometimes even 1/15, which with a fast film and a wide aperture opens up opportunities for available light pics that many other lenses don't give you (my 28mm f2 is another that's great for this). It is a bit of a weird lens though. Like most superspeed lenses the results can be a bit 'dreamy' wide open due to internal flare, though this in itself can be used to good effect. Another characteristic is that it also suffers from a bit of field curvature - ie a flat subject like a wall might be in focus in the middle but not at the edges. Can also get some distortion. At the end of the day, all lens designs are compromises - you can't have everything, but these are things you are likely to encounter with f1.4 or f1.2 lenses.
With Nikon's 50mm lenses, the f1.8 and f2 versions are usually regarded as the best behaved, possibly even the sharpest when stopped down a few stops, and definitely smaller and lighter. The faster ones are probably better down to around f2.8 but at smaller apertures there will be no discernible difference.
I always fancied the 85mm f1.4 but that's another serious chunk of money, and I managed to get an 85mm f1.8 (pre-AI but converted to AI) very cheaply a couple of years ago and it is great in its own regard, so I've given up on that fantasy. Both pretty difficult to use wide-open anyway, with razor thin DoF.
However, even though I own a few, these lenses are rarely worth the money, or the extra weight, over their slightly slower brethren, even if they are very nice to have and to use.