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I use a 1.4x II with both my 70-200 and 300 f4L IS lenses, you do take a small hit in IQ and AF speed. With my crop bodies when using center point AF BIFing is possible, it's not easy, but I don't have funds for a 500mm or longer telephoto.
A 300 + 1.4x combo (420mm) has decent reach on a crop body, you get a field of view equivalent to 670mm or so. Before getting my 300, other than zoos, the beach/pier and/or smallish ponds, I found the 70-200 + 1.4x a bit limiting, usually requiring a lot of cropping for a decent image. Even with the 300 combo there are times when I'd really like a 400 or longer, for the extra reach and faster AF. Using a 1.4x is a usable compromise.
My usual kit at kids soccer games and BIFing is a gripped 40D and a 300 with 1.4x II and a 50D with a 10-22 (a 50 f1.4, 100 mac and 70-200 are around also.) I'm looking at the 400 f5.6L, will probably rent it first before buying! I really wish Canon would update the 400 with IS or even better make a 500 f5.6L IS
There's also the 100-400 f4-5.6L IS you might consider, it's push pull design is not a fav of mine but the images I've seen are quite sharp and it's range of focal lengths are certainly convenient!
Here's a BIP (bird on pole) from earlier this year, 40D, 300 f4L IS with 1.4x...converted to B&W via Lightroom v3.6...it was a cold overcast afternoon!