hahaha what video camera do you use that writes at 20MB/s? The Canon 7D (which uses CF cards) and/or Rebel T2i (which could actually use this card) needs about 8 MB/s sustained speed TOPS. No camera out there that uses SDHC cards writes anything faster than that (yet), especially in the consumer-grade arena.
The class system is a decent way of guaranteeing that you get a minimum level of performance. it's not perfect but i've never had a major disappointment from it. however, it is nice to buy higher performance cards that exceed their class rating- I have a Sandisk SDHC class 4 that gets me 13-15mb/s sustained.