I just recently bought my T4i/18-135mm kit. I wanted a prime lens to compliment the zoom. I've already owned the "Nifty Fifty", and 'was' just going to repurchase that one, because of it's decent image quality and it's price. However, after reading just about every review out there, and viewing countless photos out there about the 40mm pancake, I decided to try it instead. I'm glad I did. The image quality completely destroys the 50mm f/1.8. Granted, the Shorty Forty is only an f/2.8, so it isn't going to give you quite as awesome background blur as a wider aperture like the Nifty Fifty....but the image quality, in my opinion, more than makes up for it. Sharp, nice contrast, nice colors, and a nice focal length to work with. I LOVE THIS LENS! I have guys that own L-lenses complimenting the photo quality of this lens. Seriously - for $200 or less.....is there really anything to think about? Not in my opinion. I didn't buy this lens for the size, because, if Canon made a cute little diminutive lens that sucked at taking pictures....big deal if it's small and compact, right? But, this lens does NOT suck at taking pictures, AND it happens to be small and compact, which to me is an added bonus. Bottom line: Unless you already have this focal length covered with another GOOD lens that takes GREAT pictures, you should buy this lens. If you're looking for your first prime, you should buy this lens. Nifty Fifty's are cool, they have their place...but the image quality of the Shorty Forty completely blows it away. Like I said...no brainer here. The two attached photos were both at f/2.8, ISO 100, 1/160s.
**Side note: For video, it's true what you've heard elsewhere: It's NOT "silent" by any means. However, if you're doing anything serious with video, you're not going to be using the cameras onboard mic anyway. I personally use a Zoom H4N recorder with a hot-shoe mount for my audio, and it works great (and doesn't pick up the focusing noise from the lens).