I've been using the 300 f4 for wildlife photography for a year now and I must say, I'm very pleased with it. I sometimes find that it's a little slow to focus and on occasion I think it's not as sharp as it could/should be, but the latter can be attributed to my tendency to forget to keep an eye on the shutter speed rather than the lens itself.
When I was shopping for a long telephoto the price tag was a big concern. I've worked with both the 300 and 400 f2.8's and knew the amazing images they were able to produce, but being an amateur, they were way out of my price range. After having read a lot of reviews I settled on the f4. I've never regretted that decision. It's light enough to hand hold, meaning there is rarely a need to lug a tripod around and for me this is huge. Don't get me wrong, I'd love to get my hands on a f2.8, but the f4 is a very good compromise.