First things first, this is not an L lense. But if you could afford an L lense, and were as uncompromising as L lense owners, you wouldn't be looking at this review now would you? What it is instead is a very well built lense that has amazing performance and construction for the price.
Having used this lense for quite a while now on a 10D, I can say that my copy, at least, is perfect for its intended usages. F/1.8 gives you the opportunity to work hand held with ambient lighting in situations where you could never get away with it's most direct competitor pricewise, the Canon 24mm F/2.8. I know for a fact a few light bulbs in a room can give you enough light to shoot handheld (i.e. 1/40) at ISO 400. Granted, it's not going to be the sharpest or the most contrasty photo you've ever taken (people have called it a dream-like blur, and I think that's quite apt). But of course the point is that you actually can get A photo when you need it.
For non-low light workers, the lens is excellent at the higher apertures. By f/2.5, center sharpness gets very good, and by f/5.6 corner sharpness is quite good as well. The lense gets progressively sharper, peaking in the range of f/7.1-13. My copy does not exhibit a great deal of "sigma yellow", though the image is just slightly warmer than the Canon 2.8.
No review would be complete, however, without discussing the physical and mechanical qualities of this lense, the most controversial judging by the comments in the reviews. Sigma has designed the 24 EX with a HUGE focusing ring, which when used is both smooth and firm, a refreshing change from the skimpy, often overly loose rings on most modern lenses. It is immensly useful in both low light, and close focusing situations, fitting the 24 EX's specialties to a T.
Now, ideally this huge focusing ring would be attached to a HSM motor with full time manual focusing. Unfortunately the 24 EX doesn't have HSM, instead using a rather loud micromotor that usually has very little to do in light of the HUGE DOF of a 24mm past something like a meter. Of course, in macro situations the motor emits a soft but rather intrusive high pitched whine (but then again, within 2 feet pointing a huge 77mm filter-size lense at something is rather intrusive in and of itself). The greatest problem with the motor is that it takes a LONG time to hunt and cycle through the entire focusing range (this probably isn't a fault of the motor, instead being a drawback of adding macro ability to a wide angle lense). In fact, in any situation where the 24 EX might go hunting, it's probably better to switch to manual and focus yourself, or you're in for an excersize in annoyance as the lense takes at least a secont to hunt around once.
Switching to manual, however, brings up the major quirk of the lense. The 2 step AF-->MF process. Note, it is not the reverse, if you REALLY need AF you can simply flip the normal MF/AF switch and the lense will autofocus just like normal, but with the focusing ring engaged and turning. This is fine for those who (like me) hold their lenses close to the body, but if you hold the lense farther out you risk your focusing motor if the motor has to pull against your grip. Switching to manual, however, requires that you both flip the MF/AF switch and pull the focusing ring back into the "M" position. This is extremely bothersome at first, but you get used to it. It even has a side effect that is really useful for a wide angle lense. If you set the focus distance in MF and then carefully switch the focus ring to "AF" mode you essentially lock the focus distance, letting the ring turn freely without losing the setting. I like this for street photography and landscape shooting, where you can essentially set the focus for infinity, lock it, and then not have to worry about it changing. Overall, I'd say the 2 step system is worth the convenience and excellence of the optics and focusing ring.
Other than that peculiar quirk there is very little else to discuss about the lense. Of course it could be better. It's big and heavy (2 times the length, and much wider than the Canon 2.8), has a large (and expensive) filter size, and it changes size with focusing (I like lenses with internal focusing, makes for a cleaner, sturdier system if you ask me). The Canon 24L is better in all these respects, and is probably optically a bit better too. But is slightly better construction and optics honestly worth 4 times the cost? If you work with an understanding of it's limitations, the Sigma is an extremely flexible and useful lense that has never failed me in what I've wanted it to do.
In conclusion, if you want a cost effective, extremely flexible 24mm prime lense that is optically very good, take a long hard look at the Sigma 24 EX. It's got some quirks, both good and bad, but it's a very good lense that won't cost you an arm and a leg.