I have used the Canon FD system extensively, including some high end(but not particularly exotic) glass, and now am pretty much fully invested in the Nikon F system for 35mm and digital. One of the things that attracts me to it is the(mostly) seamless integration across manual focus, autofocus, and digital makes life easy. I use to love my T90(one of the few FD bodies I still have), and although the F4 is a polarizing camera in a lot of ways I think it best the T90. Although it IS an autofocus camera, it has often been called the best manual focus camera Nikon has ever made, and let's also just face it that the AF isn't anything to write home about. I'm still hunting for an "A" or "K" screen to put in my two F4 bodies to make MF easier.
On the whole, Nikon lenses are excellent, although like most brands there are some dogs out there. The old 43-86 zoom is one of them(I have two, including one that I AI converted myself, and one that's still non-AI). The older Nikkor-S 50mm f/1.4s are nothing special, and are fairly soft and low contrast wide open, but the AI, AI-s, and AF(D) versions are significantly better. I nearly snapped up a 50mm f/1.4G out of the used case at the local camera store as it's a the first new optical design Nikon has introduced for this focal length/aperture in a while(barring the hideously expensive 58mm f/1.4G) but talked myself out of it since I don't need ANOTHER 50mm f/1.4. I'll also mention that the 5.8cm f/1.4, a somewhat uncommon lens that was only sold from ~1959-1962, has a very interesting rendering with "swirly" bokeh and it's worth playing with, but is also quite expensive for what it is(I'm also a hopeless collector and had to have one-I bought it on a 6.4 million F Flag Photomic). I've also used it on AI cameras, as aperture ring is cut in such a way that it will(barely) clear the tab on my D800 and D3s, but you're on your own if you want to try it yourself.
Nikon has really only made one 50mm f/1.8 that I like, the 50mm f/1.8 AI-s. I'll also add the caveat that I have not used the "G" version of this lens. In any case, I find the Series E 50mm to be a bit lacking in corner performance, esp. compared to the excellent 50mm f/2(as shown above) and unfortunately the AF and AF-D versions used the series E optical design rather than the much better AI-S design.
On the whole, though, I really like Nikkor manual focus glass. One of the things that I do appreciate about it-and any transparency shooter should-is that I find the color rendition to be both consistent across pretty much the entire range of lenses, and I find it pretty darn close to neutral. I tend to like my transparencies a bit warm in most situations, so tend to keep anything from a skylight to an 81C depending on the film stock and time of day on all lenses. Of course, I shoot films like Velvia/Velvia 50 at dusk and dawn without any filter, along with my dwindling stash of E100GX, but E100/E100G definitely needs a filter(with Nikkor lenses) to my eye and even Velvia can benefit from one in mid-day summer sun.
Someone earlier in the thread mentioned using an FM or FE with pre-AI lenses. It is true that those cameras have a flip-up AI tab and can safely mount pre-AI lenses provided that you do that, but they only work with stop-down metering. The EL2, Nikkormat FT3, F3 and F4 also have that same style tab, and Nikon can modify both the F5 and F6 with one. The F2 with DP-11(F2A) and DP-12(F2AS) also have a retractable AI feeler. Forgive me for mentioning it. but the Df also has a flip up tab, although annoyingly without a lock that can cause some weird errors if you inadvertently flip it up. AI film cameras require stop down metering to work with AI lenses, and you also lose matrix metering on the F4 and F5. I honestly don't know whether or not you do on the F6-I could see it working both ways, but I don't have the AI tab modification so can't say. The Df requires you to enter the aperture on both the body and the lens aperture ring.
A mention was made of not wanting to use lenses longer than 135mm. One of the nice things about the F mount system the beautiful little 200mm f/4, which is lightweight, compact, and excellent. Also, you can cover the entire focal length range from 20mm to 200mm with only a 52mm filter set provided that you don't want to use the 20mm f/2.8, 135mm f/2, or 180mm f/2.8(there may be a few other fast ones in there I'm missing, but all of the f/1.2 lenses are 52mm also). For me, that beats the heck out of carrying a 72 or 77mm set, as I often need to with more modern lenses, and then a set of step rings to use with the primes that inevitably also make their way into my bag.
In any case, I prefer non-AI lenses on a body purpose made for them. IMO, you can't go wrong with an F2SB, which uses the same silicon photocell of the F2AS and also the +/0/- LEDs seen on that and the FM series, but works with non-AI lenses. The Nikkormat EL gives you auto-exposure with non-AI lenses, and in manual exposure mode works a lot like the EL2/FE/FE2/FM3a. I don't like Nikkormat bodies as a general rule, though, and find having to set the aperture to f/5.6 before mounting a lens to be a bit annoying(on the F2 non-AI finders, along with the FTN for the F, you can mount with it set to any aperture and just have to be sure to spin it to minimum after mounting-I usually go max then min to make sure the follower "catches").