pixelbasher, as funhouse69, I have both the EF 70-300 f/4-5.6 IS USM and the EF 70-200 f/2.8 L IS USM, and love them both. For brevity's sake I'll call them the 300 and the 200. The 200 is definitely my 'go-to' lens when I'm shooting fast moving sports. The 300 focusing extends and retracts the lens, making it jumpier and more difficult to lock on target. The 200's internal mechanism locks on amazingly quickly and accurately. If I have time for focusing and need that extra reach, I love the 300. I think its image quality is superb, especially at f/8 or f/9.
I often shoot in low light conditions, with night matches. Can't beat that f/2.8 of the 200!
The 300 is far lighter and when I'm going to be carrying for long duration I'll tend to grab it rather than the 200. For example, this weekend, I'll be shooting motorcycle desert racing. I'll go long periods without shooting, but carrying my kit. I'm bringing the 300.
Oh, to add, I purchased the 300 first, then later saved up for the f/2.8 70-200.