Might be worth reading this before you decide?
(from:
http://www.dpreview.com/lensreviews/sigma_50_1p4_c16/page6.asp)
Overall conclusion
When Sigma first announced the 50mm F1.4 EX DG HSM in March, our story headline (courtesy of our news editor and punmeister-in-chief) was 'A new standard?', and this has turned out to be remarkably prescient. This new lens essentially redefines its class, and for once the results really live up to the marketing hype; compared to previous designs, we see significantly improved sharpness at large apertures (presumably due to a reduction in aberrations through the use of an aspherical element), and substantially lower vignetting due to that that oversized lens barrel. Chromatic aberration (both axial and lateral) has been impressively minimized, and distortion is low - in optical terms there's simply little to fault.
In short, Sigma appears to have taken a fresh look at how photographers now tend to use 50mm primes as a complement to zooms for low-light and portrait shooting, and optimised the lens to match, paying attention predominantly to high central performance at wide apertures over corner-to-corner evenness stopped down. The designers have also recognised the dominance of DX/APS-C as the current de facto standard sensor size, and ensured good performance across the frame even on this resolution-hungry format. The rendition of out-of-focus backgrounds is pleasantly smooth, again suggesting that Sigma considered portrait shooting to be an important application when designing this lens. The result is a 50mm F1.4 which is a far better portrait lens on APS-C than legacy primes designed for 35mm film, as well as an extremely competent standard on 35mm full-frame.
All is not perfect, of course; the 50mm F1.4 EX DG HSM still can't achieve anything approaching genuine corner-to-corner sharpness on full frame at wide apertures, however it does much, much better than the other 50mm F1.4 lenses we've tested so far, and it's important to realise that depth of field is so limited at these apertures that even getting the subject in sharp focus is a trial, let alone the corners (even a slight relative movement of the photographer and subject will result in a misfocused image). Also, the older 50mm F1.4 designs measurably outperform it for corner-to-corner sharpness at smaller apertures on full-frame, so if you're shooting primes for absolute image quality at F8, it offers little advantage. It's also physically quite large and heavy, as a direct consequence of all that glass needed to reduce vignetting, and therefore no longer really fits the bill as a small light prime to drop into the bag for discreet shooting; indeed it's very nearly as big as Canon's EF 50mm F1.2L USM. This in turn means it takes far more expensive 77mm filters; by contrast existing 50mm F1.4 designs make do with threads ranging from 49mm (Pentax) to 58mm (Canon). And whilst it looks like it could be the ideal fast portrait prime for Four Thirds, it appears unlikely to be able to match the Zuiko Digital ED 50mm F2.0 Macro for outright sharpness.
Of course the biggest negative currently against the Sigma is its price; it's significantly more expensive than the equivalents from the major camera manufacturers, and so the question becomes whether that optical superiority at wide apertures is worth the price premium. At the time of writing (August 2008), that's a very tough call, but it's important to understand that this lens is currently very much at the start of its product lifecycle, and if the price drops to a level much closer to the other 50mm F1.4s, then it will be quite simply a steal.
