It depends . . .
I change depending on the intended use.
For travel I like extra protection even if it adds some bulk. You would be amazed how nice the bookbook is for the mini. I had one for my 3, but it was pretty big. On the mini, however, it's kind of a perfect size. But yes, it is kind of bulky. But completely protects from dust and impact when I put it in my briefcase. Also has nicely adjustable viewing angles (landscape only).
Around the house I go between a SmartCover/Incipio Smart Feather combo or the Moshi Versacover. The Moshi, by the way, is a great cover, but they really have an odd color selection. The blue is kind of nice on a white rMini.