Hi Geoff!
If you are after my 400mm, I'll give you some advice as to whether or not its suitable.
A 400mm lens is quite 'zoomed in', it magnifies like low powered binoculars. It's not a versatile lens, by which I mean you cant really go on a holiday and shoot everything with it, just because the magnification is strong and you wont fit much in. Also it has a minimum focus distance of 3.5 meters, so if anything is closer than that the lens wont focus on it.
With that being said, if you are shooting birds or wildlife the lens is excellent, its amazing the detail you can pick out of a small bird with this lens. Excellent for any field or track sports, or motorsports.
The lens itself is very very sharp, a big step up from any kit lens. It's lighter than a 70-200mm f2.8, so not too heavy to carry around. The focusing is very fast thanks to a professional USM motor. There are two switches on the lens, one switches from manual focus to auto focus (this rarely needs touched as AF would mainly be the preferred option), and the other switch is a focus limited that stops the lens focusing on anything under 8.5m, which speeds up the focus if you are shooting far away wildlife or sports.
Have a look here for a great review:
http://www.the-digital-picture.com/Reviews/Canon-EF-400mm-f-5.6-L-USM-Lens-Review.aspx