The version of YouTube that came on the Incredible allows HQ viewing already. While watching a video, hit the menu button and "***more" and there's an option to play in HQ. The benefit of the EVO YouTube app is that you can set it to default to HQ. For me, videos wouldn't play through without serious buffering unless I was on wifi. I'm back to the original version.
Edit: With the original version, I think once you watch a video in HQ, it defaults to that until you change it back. Quality might not be as good as on the EVO, but at least it plays through without issue.