Yeah, the main thing you want is you want an adapter with a built-in microcontroller. Not just a regular plug. Those plugs that you sometimes get with keyboards and mice only work with THOSE keyboards and mice and depend on the device having a controller inside that detects what interface to use.
At work I bought these since our HP servers no longer have PS/2 ports and our 16-port KVM doesn't do USB:http://www.newegg.com/Product/Product.a ... 6812119237
(works fine on 4-port and 16-port KVM, a spare)http://www.newegg.com/Product/Product.a ... 6812156019
(worked with a cheaper KVM, not with the 16-port)http://www.newegg.com/Product/Product.a ... 6812196234
(in use, works fine on 4-port and 16-port KVM)
So these are going from USB computer to PS/2-based KVM. I left feedback on all of these (the HP DL360 G7 entries). They had the right balance of being cheap and hopefully working. Were I to get more, I'd go for the Cables 2 Go one because it worked nicely and it's in a sexy black color. Because, you know, that is really front-and-center at the back of a crowded server rack. Yup.
I do notice that the plastic casing doesn't look identical to what's in the pictures. I forget if it was the same way when I ordered before, but just minor differences. So there may have been revisions. Still, I stick by my recommendation. Buy one or two and check 'em out and return them if they suck.