Perhaps for developers it may appear out of place as we are use to complexity. But I believe for a regular user this is killer, a simple spaced out flat design helps make the decision making faster. Again it depends upon how well you use flat ui theme. In flat UI I believe minimalism and spacing is very important.

I recently started using windows phone & win 8 pc. After using it for sometime I have realised that if you are into only few apps & want to use it effectively & productively then this is a great UI. These UI do not encourage more apps usage.

So next time if you are building a simple tool, flat design might be really cool. It makes users believe that there is less effort & it is faster. Again you need to master it to take good advantage of it like very other style. Signup page using flat ui is also a great idea if you ask me.