I've had my eye on this lens for a few weeks and finally bought it at a brick-and-mortar store in NYC (thus the lack of �verified purchase tag). I'm using this on a 5D IV body. For comparison, the other lenses I'm most familiar with are all Canon lenses: the 24-135mm 3.5-5.6, the 24-70mm 2.8 II, and the 70-200mm 2.8 II.PROS:Overall, this lens is a ton of fun to shoot with. It's my first prime, and I've really enjoyed the process of taking pictures with a prime vs. a zoom lens. Some people may not like it - and there's a lot that you're going to give up compared to a zoom - but the prime makes the process of composition slightly more challenging and interesting in my mind. The focal length is perfect with a full frame camera: the process of composing a shot is as simple as looking at your subject because the 40mm length very closely approximates what you see with your own eyes.In terms of the actual lens performance, image quality is outstanding, as would be expected with a prime. This is especially true given the cost of this lens. There are no significant optical aberrations that I can see on the images that I've taken - again, largely unsurprising with a prime lens. When in focus (more on that below), images are incredibly sharp and rival the sharpness of the 24-70. The size is outstanding and really makes the system way more portable. With the 5D IV and an attached battery pack with two batteries, the body itself is quite heavy. The 24-70 and 70-200 are both massive lenses, so the whole system is quite unwieldy and heavy when using one of those lenses. Not so with the 40mm - the lens is extremely light, and the pancake form factor is awesome for street shooting. Bokeh is decent and comparable to the 24-70 to my eye, and the wide aperature makes shooting in low light a breeze. Compared to my other lenses, the motor is certainly louder but I don't find it unacceptably loud. Perhaps in a quiet setting it would be problematic, but for all settings that I've used it in I haven't noticed the noise. The only other downside - like the noise, not worthy of a lower rating, but simply something to be aware of - is the relatively limited number of cross-type focus points. All 61 points on the 5D IV are usable, but only about half are cross-type points, and only one column of points on the sides of the frame are cross-type. Again, not a huge problem, but something to keep in mind.CONS:My only serious complaint about this lens - and, thus, the reason for the 4 stars - is the completely unusable speed of the autofocus. Slow is an understatement - this lens takes a good 2-3 seconds, at least, to acquire focus using the autofocus system. I use manual focus exclusively with this lens and simply have the camera tell me when focus is achieved. This is actually much faster than the autofocus system, and the only situation I can imagine using the autofocus is with a perfectly still subject. Because of this, this lens is completely unusable for subjects that move with any degree of reasonable speed unless you're quite good at achieving manual focus quickly. On an open air bus tour in NYC, I had trouble taking pictures of any subjects with this lens as the bus was moving because of the difficulty with focusing. I had to pass on many shots that I could easily achieve with the zooms that have laser fast autofocus, and I ended up switching over to the 24-70 in the middle of the tour as a result. I had read about the speed of the autofocus and did expect some slowing down, but not to the degree that has actually been the case. Keep this in mind prior to buying this lens.OVERALL:Slow autofocus aside, I still absolutely love this lens, and it has stayed attached to my body almost exclusively since I bought it. Obviously it's not the ideal lens for all scenarios - and I'm still going to keep the 24-70 and 70-200 glass in my camera bag - but for day-to-day shooting, casual shooting, and street shooting, the 40mm is my go-to. It's a bargain for the image quality, and the focal length is perfect, particularly on a full frame body. So glad I ended up picking this thing up!