Should I buy the canon 100 400mm
Of course you should! If you don't need a fast lens, this is the best value Canon deal there is, in that FL range. Just bear in mind that at 400mm, this lens is sharpest at f11. That can be a disadvantage if you don't have a camera with low noise at high ISO's. I use the 20D so there's less problem in that regard. I consider my 20D has upgraded my 100-400.
Some people wonder if they should get the superior 70-200/2.8 instead and use a 1.4x or 2x extender. Depends on what range of focal lengths you think you'll mostly use. I'd suggest most people buy the 100-400 to use at 300-400 rather than 100-300.