I got one, only because of the weight weenie factor (and I like the idea of no grease). They probably are less efficient, and thus you'd be losing some watts.

What's more, the weight savings are overrated, as they are dependent on using a heavy chain and chainrings. If you select a relatively light chainring and chain the weight difference is as little as 112 grams. Weight savings that small are nothing to be losing watts over.