Please note that this review is by an amateur, who is starting to get into photography - an experienced photographer will probably read it and think 'well, of course, what did you expect?' when they see my initial struggles with it, but people who are like me may find some help in reading of the issues to help them decide. Reading about it in books isn't the same as actually experiencing it.
This is a very imposing lens, I was rather shocked to see the physical size of it when I opened up the very well packaged box. It comes in a well padded carrying case that will protect it any time it is not fitted to the camera, and it also comes with a large hood which can clip to the front of the lens in reverse for storage and transit.
I fitted it to my EOS 500D, after a couple of seconds of panic when I thought it wasn't compatible. It is very big on the 500D, and the lens on the end is a very large diameter indeed; be prepared to spend a lot on filters.
I bought this lens primarily to improve indoor and low light shots, but I showed my lack of experience with sophisticated cameras during my initial attempts with the camera. Be warned that at large aperture settings, the depth of field is paper thin. I struggled to get the focus right when using the camera on auto focus settings, and it really doesn't give you much tolerance. I also struggled with the exposure initially, with the photos coming out over exposed. However that was the point of the having some test sessions, to let me make the errors when they didn't matter.
Once I had played around with the lens for a while, I realised that the largest aperture setting is one that is only really to be used in very specific circumstances. Once I stopped back to f4.0 and beyond, I found things got vastly better for me. I had sort of got myself into the mindset of 'why have a lens that can open to F2.8 and not use it', but of course the size of the lens means that even at the smaller apertures, the actual aperture was bigger than the basic Canon lens it was replacing. This meant that I could use a faster shutter speed than before, and suddely all came good. The depth of field obviously improved, and the lower light shots suddenly were much better.
Using the lens on the fully automatic settings of the EOS wasn't such a successful story unless you were shooting during daylight hours. Indoors, I found that the depth of field issue was quite prevalent, and the autofocus was not perfect. Putting the camera back to manual settings helps this, and making sure that the aperture is closed down results in a greater chance of getting the photo right.
I have to say that in reality, if I was going to use the camera for indoor shots where I needed the assistance of the auto exposure and autofocus together, I probably would put the Canon lens back on. Maybe as I become more adept this will change, but beginners like me should be aware.
However, it is a sophisticated lens, and when I have managed to get it right, it has provided some fantastic pictures. I have portrait and posed photographs of my wife, and the ability to blur the background out, and be more creative with lower mood lighting has been very enjoyable. More experienced photographers can tell you about vignetting, chromic aberration and so forth, I don't feel competent enough to be able to offer an opinion yet.
I'm still on the foothills of the mountain of experience that I will need with this lens (and camera, to be truthful), but I have seen enough to make me feel pleased with the purchase, and am thoroughly enjoying using the camera with this lens.
As I get more experience I will edit this to reflect it.