They aren't cheap but we run a pair of Kuhn rotary rakes and I know for us they have paid for themselves.  We've ran wheel rakes and bar rakes in the past and the rotary does a cleaner job with a faster drying time and in small fields they get around much better than a wheel rake.  We started with the rotary rakes when we were baling a lot of small square bales for horse hay and consistant windrow paid off in a premium quality bale, for cow hay I don't know if a rotary rake would be cost effective but once you get used to one it would be hard to go back to anything else.