Admittedly, I'm a relative beginner to this level of equipment. I'm used to my lenses having image stabilization (IS). This lens does not. Many of my hand-held night shots were blurry due to a combination of camera shake and slow shutter speeds (its a relatively slow f/4 lens).
I ended up returning the lens for the 24-105 f/4 USM with IS. Amazing difference in picture quality.
I'm sure the pros can get outstanding image quality from either lens, especially with a tripod at night. But I need IS for my typical shots, which are hand-held, in poor light. The IS gives me about 3 full stops equivalent advantage regarding hand-shake. (nothing for subjective movement of course)
So, my recommendation; try this lens in your most difficult anticipated situation before you buy it. Or, buy from a place you can exchange.

It's around US$729.99 in Amazon.com

I have been using canon equipment for donkey years now and was just about to buy this lens when a very helpful staff in my usual photo shop told me about the problem with the diaphragm. It very often gets stuck, but not within the first year of use, sometimes in the second or after the second year of use (when you have no more warranty).I then went to the repair centre and spoke to the head engineer and he confirmed the problem. What do you do when this happens? YOu need to get a new diaphragm - usually out of your own pocket. Canon has apparently addressed the problem and came up with a new diaphragm, but the engineer said it was only better than the old, but still not perfect. Apparently these 'new diaphragm cameras' are mixed in with the old ones and so there is no way of knowing what you buy. the dealers don't know either which is which. Now, this put a big damper on this lens. No matter how good it is when it works, it is only as good as its weakest link. shame on Canon! I just had a problem with my 5D...

It's around US$1055.95 in Amazon.com

I think the first thing that must be said is that this lens takes great pictures. The IS works really well and allows hand-held use, even in relatively dim light, if you are willing to crank up the ISO enough. I have taken some pictures I was very pleased with at about 7pm in the evening by using ISO 800 and, upon occasion, ISO 1600. Otherwise this lens needs either a monopod or, preferably, a tripod.

The barrel slides in and out smoothly making it easy to frame your picture and the auto-focus works well. Fast and quiet. I have not had a problem with the AF hunting except in very low light. Except at 400mm the pictures are sharp and clear and, when the lens is fully open, the DOF is nice and slim allowing the subjects of the photo to really stand out. Even at 400mm this lens is much better than my Tamron 200-400 in terms of clarify and, what softness I did find at 400mm, was easily corrected in Photoshop. All in all a very nice lens except that ...

1) This lens is heavy. I know other p...

It's around US$1610.00 in Amazon.com

The only lens you need to carry when traveling. The choice was between a two lens solution of a 24-70/f2.8 + 70-200/f2.8 or just this one lens. I rented the 70-200/f2.8 and this lens to try them back to back. The weight differential is not that noticeable. The choice came to lower overall weight of the one lens solution over the two lens solution. The compromise was in losing the f2.8 capability. I shoot with a 5D mk II and already have a 24-70/f2.8, the plan was to use the 24-70/f2.8 for indoors/lower light and the 28-300 as the outdoor/bright light solution. However knowing my intense dislike for changing lenses. It turns out that I am seldom using the 24-70/f2.8 anymore. For recent travel I just carried the 28-300 lens as the sole lens and addressed the low light requirements by carrying the very compact EX270 flash Canon Speedlite 270EX Flash for Canon Digital SLR Cameras

The 28-300 is definitely heavy but it is still lighter than a camera bag full of lenses. I have done away with the 5D strap...

You can buy this in Amazon.com