HTC's stock video camera has a pretty impressive compression rate, so you have little chance of filling up the buffer before it writes the video. Use a market camera app that supports 720P and has a shaky compression rate and see how long it takes to crash the app. It took me about 23 seconds...
It also has to do with how full the SD card is. The more data on the SD card, the more likely your card will ONLY have small groupings of data blocks available for the app to write to, this takes longer and can cause the memory buffer to fill up faster, bringing you back to my first point...
Many people use a class 4 and are happy with it. HTC and 720P get along fine with a class 4. Just make sure you use the stock camera and keep an eye on how full your card is and you should be fine.
Just because it's never happened to willis doesn't mean it's never happened...