[Left to right: Canon's EF 50mm f/1.2L USM, 50mm f/1.4 USM, & 50mm f/1.8]

Canon has three versions of a 50mm prime (non-zooming) lens with vast differences in price, features and quality.

All of these models share these features:
* Auto-focus (of varying speeds and qualities)
* 46° Diagonal Angle of View
   (equiv. to 30° on an SLR with a crop-factor like the XTi or 30D)
* 50mm Focal Length
   (equiv. to 80mm on an SLR with a crop-factor like the XTi or 30D)
* 1.5'/45cm close-focus distance
* Large aperture compared to most zooms
   (smaller "f/" numbers equal larger aperture which means more light)
* Excellent depth of field
   (ability for high background blur, useful for portraits and closeups)
* Compatible with any Canon EOS camera, film or digital, full-frame of crop-factor.
* Canon quality, reputation and warranty.

Key differences between them:
EF 50mm f/1.8
*Inexpensive
*Very lightweight and compact
*lower quality (plastic mount-ring, etc.)
*Lens changes length while focusing
*slower focusing system (compared to USM)

EF 50mm f/1.4 USM
*Superb quality (metal mount etc.)
*f/1.4 speed makes it better for available-light shooting
*UltraSonic Motor for very fast, accurate and silent focusing
*Compact compared to most zooms
*Micro USM with full-time manual focusing override (rare)
*Lens changes length while focusing
*Middle-of-the-road price

EF 50mm f/1.2L USM
*Peerless EF lens
*Ultra-large f/1.2 aperture for a narrow depth of field and soft background blur
*Consummate available-light performance
*Lens coating and construction are optimized for digital cameras
   (to minimize the ghosting and flare)
*High-performance, weather-resistant lens
   (Canon's "L" series are dust and water resistant)
*Superb image resolution and contrast
*Internal focusing
   (lens does not change length while focusing)
*Lens hood and pouch included
*Very expensive
*Fairly large and heavy for a prime lens
*"Street cred"

EF 50mm f/1.8 - $75 (street price):
Focal Length & Maximum Aperture
    50mm 1:1.8
Lens Construction
    6 elements in 5 groups
Diagonal Angle of View
    46°
Focus Adjustment
    Overall linear extension system with Micromotor
Closest Focusing Distance
    0.45m / 1.5 ft.
Filter Size
    52mm
Max. Diameter x Length, Weight
    2.7" x 1.6", 4.6 oz. / 68.2 x 41.0mm, 130g

EF 50mm f/1.4 USM - $275 (street price):
Focal Length & Maximum Aperture
    50mm 1:1.4
Lens Construction
    7 elements in 6 groups
Diagonal Angle of View
    46°
Focus Adjustment
    Overall linear extension system with USM
Closest Focusing Distance
    0.45m / 1.5 ft.
Filter Size
    58mm
Max. Diameter x Length, Weight
    2.9" x 2.0", 10.2 oz. / 73.8 x 50.5mm, 290g

EF 50mm f/1.2L USM - $1300 (street price):
Focal Length & Maximum Aperture
    50mm f/1.2
Lens Construction
    8 elements in 6 groups
Diagonal Angle of View
    46° (with full-frame cameras)
Focus Adjustment
    AF with full-time manual
Closest Focusing Distance
    1.48 ft. / 0.45m
Filter Size
    72mm
Max. Diameter x Length, Weight
    3.4 in. x 2.6 in./85.4mm x 65.5mm
19.2 oz./545g (lens only)

Well, balance you needs with your budget. If you have an inexpensive film camera or an entry-level DSLR, The f/1.8 is a great deal. It gives you a very bright, compact, easy to use lens that gives great results. If you ever feel the need to upgrade, they hold their resale values very well on eBay. If you have a middle-of-the-road DSLR, like the 20D or 30D, the f/1.4 USM is worth considering the extra $200 get you a brighter lens, better quality and faster focusing. If you have a high-end SLR or DLSR like the 5D the f/1.4 USM will give you very good results, but if you have already invested at least $3000 in a camera system, you may want to look at the f/1.2L. It is simply the top of the line. No matter what you should consider how often you are going to use the lens, if you never shoot in low light or don't like using a non-zoom lens but are just looking for something to keep in the bag, just-in-case, the f/1.8 will suit you needs and keep your wallet happy. If you are a portrait photographer or constantly shooting with available light, the f/1.2L may be worth the investment, especially if you rely on your gear for an income. the good news is, no matter what you choose, these lenses hold their value as long as you take care of them. Keep it protected and clean and, if in 6 months you find you just are getting the use out of it that you wanted, sell it on ebay at a minimal loss.