Pros: Small and compact. Good contrast and clarity in the middle at close distances and narrow apertures (F4+). Not too much CA or distortions. Decent build quality and nice crew-on hood design. No AF problems. Price used.
Cons: Feels dated optically, corners are very soft up to around F5.6. And it's never sharp enough for landscaping. Canon didn't update this lens for more than two decades, but they keep selling it at a rather high price (for what it is) with the hood sold separately. Some people like this old lens "character" very much, some people don't. For me it can be pleasing at times and, honestly, that's why I bought it. Wide open it produces weird/interesting looking bokeh as well as some tricky color aberrations and it flares like crazy too. Many would consider it to be unusable in F1.8-2.8 aperture range, which is only fair.
Opinion: Got mine used for $250 (lens hood included), which I think was a very good deal. I knew exactly what I'm getting into :) and I have no regrets. I love using it for close-ups, portraits and parties on my 6D. It is great for forest hiking or street photography as well. The 28mm field of view turned out be a very good compromise between the overly distorted 24mm and the not so wide 35mm. I've learned that the focus distance has major impact on the image quality of this lens. Basically it makes my camera "short-sighted". Anything that's just a few meters away is nice and clear, but not too much :). Anything farther away turns into various degrees of blurriness, no matter what aperture I'm using. However, I think that most people are much less picky about image quality than me. But then most people are using APS-C cameras and I would strongly recommend getting the 18-35F1.8 Sigma lens instead.
Pros: Interesting bokeh at F1.8, kind of sharp enough in the middle, which is fine for portraits, but rather soft overall wide open. Quite usable though. Not much CA. Sharp from F2.5, excellent from F4. STM - silent and faster AF than in the older EF 50F1.8 II lens. Now with 7-blade aperture and closer focusing distance. Small. Weights almost nothing. CHEAP!
Cons: Odd 49mm filter thread, down from 52mm on the older one. STM is focus by wire, which means that when the battery dies, you cannot "park" the lens front element back inside and it will be sticking out. Flares badly, but some (?many) people like it very much.
Opinion: This one is simply the best value lens, period. A must-have Canon prime with good old (traditional) Double-Gauss optics in a new plastic barrel. Well, why fix something that isn't broken ... Right? :). Unless you want to compromise it somehow. Some say that this new STM version has worse vignetting than the older lens. I didn't check it myself, because I don't find it to be a problem really. Still, it might be worth mentioning. If I had to choose between 35L on APS-C and this 50 STM on FF, I would take the 50+FF.
Pros: Very good image quality - sharp, good contrast and bokeh. Light, compact, fast and silent AF with FTM, popular 58mm filter thread. And all that for less than $400.
Cons: Color aberrations (the fringing) can be too much sometimes, in some specific lighting conditions. Disappointing lens hood design, and it's not included. The minimum focusing distance is 0.85m and it doesn't make it a good close-up lens.
Opinion: In short, for the price, it's a dream lens. A wonderful option for someone starting his FF adventure. The moment I put it on my 5D2 for the first time it was like magic. I could shoot a nice, or seemingly flawless picture of anything and everything with it. Almost no boring images. I even kept many misfocused ones. Compared to a cheap 50F1.8, this is a next level lens. And it's the level that I'm comfortable with to this day. However, if you have an extra $100 lying around, then you might like its bigger "sister" more, the EF 100F2 USM, which is a bit better in many aspects, same level though :).
Pros: Great image quality. Nice bokeh. Sharp at F2. Decent build, better than the EF 85F1.8 USM (a bit more metal). Fast and silent AF with FTM. Pretty compact and not so heavy. Quite affordable. Small and popular 58mm filter thread.
Cons: The hood design is a joke, which is a rattling piece of plastic and it's sold separately. Color aberrations (the fringing) are lower than with the EF 85F1.8 USM, which come into play only in some specific lighting situations, but they are still there. Not a big problem (for the price). Doesn't work well for close-ups (MFD=0.9m). Not stabilized.
Opinion: I really like this lens very much. More than its smaller, but slightly (and reasonably) cheaper 85F1.8 "sister". Works beautifully on my EOS 6D. I wish it could focus a bit closer, but there's nothing that an extension tube couldn't fix :). It produces pleasantly sharp and contrasty images with a nice bokeh. Which is a fantastic deal for $500.
Pros: Full Frame. Image quality. Intuitive ergonomics and menu. Juicy LP-E6 battery, shared among many prosumer EOS cameras. Smaller and lighter than 5D series. WiFi, GPS. Good high ISO performance. Silent shutter
Cons: AF system not much better than in the old 5D Mark II. Despite many design similarities with EOS 60D/70D/80D, it lacks a vari-angle touch screen. SD card slot - I'd really prefer CF (or two).
Opinion: The 6D in many ways is an improvement to the 5D Mark II - better sensor, better plastic (no rattling doors), faster shooting, WiFi, GPS, better battery life (same battery though) and the smaller size and weight is appreciated by many. The back wheel+joystick/dir-pad combo works nicely. Now I can easily operate with my right hand without taking it off the grip, while holding the lens with my left, because most of the buttons are on the right side. There are some little things, like 1/4000 maximum shutter speed, 1/180 flash sync speed or no dedicated WB button, which make it inferior, but, for me, they are not that important.
Pros: Full Frame. Image quality. Intuitive ergonomics and menu. Juicy LP-E6 battery, shared among many prosumer EOS cameras. CF card slot. mRAW, sRAW. Rugged construction. And a vast EF lens family to play with.
Cons: Shadow recovery isn't that great. Consumer grade AF. Only 3.9fps. Fixed LCD. And a bit heavy.
Opinion: It's a great tool for many deeds. 22mp in its time was the state-of-the-art imaging sensor with 1080p video/LiveView features and to this day those pictures hold their ground well. Nice LCD quality and size, but it doesn't really work well in bright sunlight, which is only normal, I guess. Now, being a decade old camera, it's a fantastic camera to start with for any beginner, photography enthusiast, because it can be had used for less than a brand new Rebel class DSLR (less than $600), which is crazy and fantastic. IMHO, CF card is a faster and much safer media to deal with, than SD, but, if needed, SD can be easily adopted via CF-to-SD adapter.