Here's my two cents and some basic photo knowledge.
I've shot with both the 50mm 1.4 and the 85mm 1.8 (on full frame cameras), but prefer the 50mm because I like to capture environment along with my portraits. 85mm is a great portrait lens and is pretty sharp (sorry I don't know how to quantify the sharpness). The 85mm is commonly used for portraiture. If you're a casual shooter and trying to decide between this and the 50mm, I'd suggest the 50mm. (But really, I'd suggest getting a zoom lens rather than a prime if you're a casual shooter). 50mm is somewhere between wide and telephoto, so it's easier to move in or away from your subject than it is to do with an 85mm. That said, it comes down to is what and how you shoot.
If you're shooting with an APS-C body (Rebels, XSi, T3i, T4i, 60D, etc), multiply the focal length by 1.6. IE: 50mm x 1.6 = 80mm. So yes, slapping on a 50mm lens on your APS-C body will be like slapping on an 85mm lens. You have to do this because the sensor on the APS-C bodies are smaller than the full frame bodies.
Here's my lens usage suggestion:
17mm-40mm Great wide zoom range for landscapes, tight rooms, etc.
24mm-105mm Great zoom range for everyday shooting. Gives you some ability to shoot wide and long. Fine for indoor shooting. Probably ok for some landscape.
70mm-200mm+ Great for when you want to capture tight shots, but don't have the ability to move in closer. Sometimes not ideal for tight spaces.
But again, it comes down to what you shoot.
Hope that helps someone.