The buffer in my 20D is pretty good...so I've never had a problem while shooting...granted, I don't shoot sports.
Like I said, I did find that the slower cards are slower when you want to bring up the image and/or histogram on the screen. The initial review was OK...but if you shoot a bunch of shots and then want to quickly review them...the slower cards are noticeably slower. I might not have even noticed at all....but when I replace a fast card with a slower card...I found it quite annoying.
For $20-$30 more...I will opt for the faster card. I usually wait for sales and rebates and just get whatever is on sale anyway.