I tried this lens out when looking for a low-cost telephoto lens for my Digital Rebel XT, side-by-side with the USM version, the IS version, and several L-series lenses in similar focal lengths (the latter being but a pipe dream and way out of my budget at the time).

In short, if you want a lens with a long reach and have next to nothing to spend, this might be the lens for you.  I'd suggest spending the extra money on the USM version for the slightly-faster focusing--it's really just a pittance more, unlike the IS lens (although not appreciably sharper than this lens).

Photos are remarkably soft, particularly toward the 300mm end.  If you want this lens for nature and sports photography, be prepared for a lot of work in Photoshop to get something sharp enough to use (and forget about professional-quality prints).

The focus is very slow, and the lens itself rather slow throughout its range--and particularly at 300mm, where you're going to be locked into f5.6.  This means longer exposure times and/or requires better lighting, which limits its usefulness for capturing action at any distance.

Personally, I'd save up a bit more or else rent a better lens if you immediately need something in this focal range.  However, if you're okay with the softness--perhaps you're only doing portraiture and want to put your camera halfway across the room?--and can deal with the other limitations, the lens is very affordable.
I tried this lens out when looking for a low-cost telephoto lens for my Digital Rebel XT, side-by-side with the USM version, the IS version, and several L-series lenses in similar focal lengths (the latter being but a pipe dream and way out of my budget at the time). In short, if you want a lens with a long reach and have next to nothing to spend, this might be the lens for you. I'd suggest spending the extra money on the USM version for the slightly-faster focusing--it's really just a pittance more, unlike the IS lens (although not appreciably sharper than this lens). Photos are remarkably soft, particularly toward the 300mm end. If you want this lens for nature and sports photography, be prepared for a lot of work in Photoshop to get something sharp enough to use (and forget about professional-quality prints). The focus is very slow, and the lens itself rather slow throughout its range--and particularly at 300mm, where you're going to be locked into f5. 6.  This means longer exposure times and/or requires better lighting, which limits its usefulness for capturing action at any distance. Personally, I'd save up a bit more or else rent a better lens if you immediately need something in this focal range. However, if you're okay with the softness--perhaps you're only doing portraiture and want to put your camera halfway across the room? --and can deal with the other limitations, the lens is very affordable.