Canon EF 100-400mm f4.5-5.6L IS USM Telephoto Zoom Lens for Canon SLR Cameras

Pros: Focal Length, ease of use- push/pull is very intuitive and smooth.

Cons: There is a learning curve with this lens! Not weatherized.

Summary: If you want a 400mm+ lens that features image stabilization and you can fairly easily carry- without another mortgage on the house- you should consider the Canon EF100-400mm f/4.5-5.6L IS USM lens. It does take some getting used to with it's push-pull zoom function, but once you do get the feel for it- you will appreciate it's ease of use.
This lens isn't the fastest lens around, and it has the older stabilization system- so you do sacrifice a little with this lens. However- it is thousands less than many of it's competitors, so you should expect to "give a little."
Ultimately, this lens is a lot of fun to use, and it can really reach out there and get photos that would otherwise require a lot of crop. So, if you have the opportunity to get out there and play with this lens- you won't believe how it will bring a smile to your face!