IMO, flat design, when skilfully used, can look really good. Of course there is quite a bit of bad flat design out there, just as there is of any other design style.
And don't worry, trends cycle with time, so it stands to reason we'll be seeing some more depth on our screens again some time in the not-too-distant future. (Depth of design, that is, not necessarily depth of content ;-) )
Historically flat design is a form of minimalist design articles point to it being rooted in Soviet design styles.