I have written one HTML5 game http://magigames.org/runestone_defense.html and I am working on another.
HTML5 runs fine on a PC, but is too slow on my iPhone 4s and my iPad.
I chose HTML5 because I wanted to brush up on my Javascript. If I wanted to make $, I'd have chosen Flash.
Making sure everything works in various browsers / OS is not too bad. I test in IE9, Chrome and Firefox on Windows 7, Chrome, Firefox and Safari on Mac, and Chrome Firefox on Linux. It can be time consuming to try them all, but once I nailed down the differences (mouse events in IE, most notably) it wasn't too bad.