Having decided you need a 500mm lens, the next question is do you want to spend the equivalent of a meal out on it, or the equivalent of a second hand car. If you go for the latter, then go and buy yourself a good canon lens like Canon EF 500mm f/4.0 L IS USM Lens
and you won't be disappointed. If your budget is a bit more restricted then read on.
This lens is well constructed and is very solidly made. It comes with three filters which are inserted at the back of the lens, between the camera and the lens itself, not at the front. Notice that as a result of that if you want to protect the front face another filter will be required. Your camera will not be able to control the aperture setting which is fixed at f/8 so you will be using predominantly Av mode. With our Canon 400D that largely worked ok but there were occasions where some shots were over exposed and it wasn't clear why that was happening.
The key limitations with this lens that you need to be aware of are as follows:
1. Contrast will never be as good as you might hope, this is as a result of it being a mirror based system. If you are taking shots of the moon this will not be a problem but nature shots (for example) will tend to look a little more washed out.
2. You will see an effect called "bokeh", which is where the out of focus background tends to have lots of circle effects.
3. If photographing nature, the narrow depth of field and manual focus means that you really need to choose an area to focus on then be prepared to shoot when the animal comes into shot. This is a great deal different from the normal point and shoot approach that you may be familiar with, where the auto-focus can be used to good effect.
If you are happy with the above, this is a very cost effective purchase.