Feels really nice actually. I had an S2 for 2 years and 7 months before. The 5's silky yet matt back feels premium. The weight is not so light that you'd think it's cheap, yet not too heavy that you'd think it's been poorly made, it's just right. Only gripe is the edges (from glass front to the sides) is more square than the likes of my curvier S2, it digs in a bit if its in my pocket and I lean forward. My little fingers rest on the corners in such a way when I type that I have indentations on them after prolonged use... but that's just something I've noticed. The power button is higher up, near the top and requires more reach. But that must be because I had my S2 for almost 3 years and was naturally used to its position.
Summary - feels lovely and premium.
Edit - to comment on your topic of "grip" it is almost rubbery on the back so I've no worries about it slipping out my hand at all. You should be holding your phone with a firm grip anyway, regardless of backing material!