I got mine well before the 450D was released, used it on my 450D and now still use it on my 7D; only reason I'd say to go without it and get a Sigma in the same price bracket is that f4 is high action in low light, such as gymnastics in poorly lit indoors, can be frustrating, still very possible, but frustrating; and it was designed for the film days so you can get hot lights (like looking straight into car headlights at night) hitting the sensor, reflecting back of the rear element so it's a little green ghosting on occasion, again in extreme lighting conditions; that said 99% of the time this is an awesome lens, portraits, outdoor sports, hiking, etc. I've taken at least 100,000 images with it. It's robust as well, 6 years and the UV protecting filter needs replacement (threads got bent by another filter) and it's picked up a few minor scratches, but it's been my go to lens for 6+ years.
It is heavier than it's EF-S counterparts but I personally think there is a quality had for it; and it's lighter than the 2.8 70-200s so even if you add those later, many keep this one because it is light weight, sharp and has great IQ. I've used the IS version, I like this one better.
I got mine new from B&H and was worth every penny for me, if you find it cheaper than that and it's in good shape then go for it. It was also my first L lens, which completely re-adjusted my standards in terms of lenses; so while you may think you're getting a deal now, you'll be saving and spending more down the road for the L quality.