Yeah wildlife photography, particularly bird photography, can never have enough focal length. I practice stalking techniques to get closer (which has a much better cost/reward ratio to buying bigger lenses) but if you can fill the frame with a bird you can get awesome shots that just aren't possible otherwise. Currently I'm shooting at 600mm, and I definitely want a teleconverter to get up to ~800mm. After that the prices get truly insane so I think I'll just have to find some bigger birds at that point.