This is really nice, but I've always enjoyed RegexPal [1]. It's got a nice clean simple UI, displays results immediately and allows me to prototype regular expressions very quickly. Use it almost every time I wrote a regex.
The problem is, regex flavors differ, and RegexPal uses JavaScript - whose regex flavor is really limited compared to most others (including Python). You should always try to find a tool that uses the regex flavor you are targeting.
[1] http://regexpal.com/