I rarely use the lens wide open; I prefer F6.3 or F7.1. There are alternatives - Sigma do a couple of them. The Bigma is 50-500mm, with no IS and is significantly heavier. But is a good lens, and has a wider range. And there's a newer 150-500 lens with IS, but a friend recently chopped his in for a 100-400mm after seeing pics I got sat next to him!
Another popular lens is the 400mm F5.6 prime. Light weight, fast AF and great for wildlife. No IS, but doesn't seem to hinder it. Cheaper too. But you lose out on the flexibility of the zoom, and the minimum focusing distance is further, so can be a pain when photographing insects, for example.