>>3374571I've been shooting with a D5100 for 7 years as my main rig. I installed hacked firmware for uncompressed RAW files and HD video and it makes a huge difference in quality. Overall it's been very good to me, and I use almost exclusively non-AF and in some cases pre-Ai glass, no problems. It will say "No lens attached" when you try and use old glass in any mode other than "M", but you can use pretty much anything on it. The lack of TTL metering sucks, my old D1x can do it with full manual lenses and its really helpful, but the D5100 is pretty limited. I find you get used to what exposure it can handle though. It's got pretty good low-light performance, which is made better with wide old glass.
If you're upgrading, The Fujifilm X-T1 looks like the nicest sweet spot for the $ due to being able to use any old lens with mirrorless. The D7100 isn't bad either since you can find them for under $400 these days and the image quality really is great. The D700 might be full frame but it's old and you will definitely feel its age when you use it. Don't get an A7 unless you can afford to get the mkII.
Sample shot from my D5100 using a 1961 Nikkor-S 50mm f/2
