I really like the idea of the constraint validation stuff that’s built in to HTML5. I’ve never actually used it though, since it’s not going to work in older browsers. We tend to use the standard ASP.NET validation controls on most of our projects at work, and they’re certainly usable.
The article I linked to above has a section on a couple of options for polyfills, allowing the constraint validation to work with older browsers, though, so maybe I’ll give that a shot the next time I need to do some serious web form stuff.