I use the 70-200 f4 L on my XSI. It was my first L series, and we tried a 100-400 before that and there was just no comparison in the movement of the lens.
This lens is a great way to get into the L series. The cropped sensor is going to give you a little more reach, won't it? I thought it worked out to really being 112-320 when the 1.6 crop factor is factored in, but I will leave it to the others to jump all over that if I'm incorrect.
If you like to use a monopod or tripod, the ring mount collar is a good idea, because that lens is going to really unbalance a smaller camera. The ring mount cost us an additional $140. For the way I use the camera, I really needed the collar. Others may not.
I'm very happy with the 4.0 in daylight situations, particularly in strong sun. I have significant enough trouble in lower light that I often wish the 2.8 had been possible. An eventual body upgrade may help with this, but that is years down the road, and I foresee many more years of me swapping out to my 50mm just because the light is too low for the 4.0.
I can wear the 70-200 mounted on the XSI very comfortably on the Cotton Carrier harness while hiking. It fits standing up in my messenger style camera bag when I wrap it for travel, and with the smaller body size I find it relatively easy to stow in a horizontal position inside the bag for short periods of time. I do feel the camera is a little off-balance when this lens is on, so I don't like to travel with it on.