Hi, your site is really simple but very useful. But let me say, I think you need bring ranking info table stay upper (or under) the ads panel. It seem to be better.
I've used a lot of sites similar to this one. To be honest, most provide more information. There's plenty of places to get this kind of information. You're seems to work well, it pulled my PR correctly and the Alexa and back link numbers are close enough to seem correct. So functionality you get an A-. You could add more features like social bookmarking links from other sites.
The header needs help.
I clicked the Rank button 4 times because I didn't think the site was working. It took no time to receive my ranks (good thing!), but they were off the screen. I didn't see them come up so I kept pressing. I think that's a real issue you'd want to address. If the results were above the fold, I wouldn't have had that issue. If it's a question of AdSense position, move your AdSense to just under the header. Move the header Up toward the top of the page and put the results just under the Rank button.
It looks good and it pretty useful. That's the important think. Maybe try changing your google link color to an orange. It will look more like part of your site and you may get more clicks.
