I saw one on a friend's K the other day, and now I kind of want one. I see that they are discontinued, so how badly would I get screwed if I wanted to buy one? My friend mentioned he would consider selling it but he didn't say for how much.

Anyone have one that they have an opinion on?

I’ve never used one, but I had a couple surefire ar15 rails and I’ve never loved them.  I feel like the rail is not truly m1913 spec.  They are also sharper than say.. the kac rail.  I just don’t love them.

I don't have the M77 for the MP5K, but I have 2 of the M63 for the fullsize MP5 and like them very much. Unlike the KAC RAS, you can tighten them down to fit any variation in specs on your specific gun. It's best to use LocTite on that adjustment screw and then leave it on the gun, but you can QD them also, if you want (which is unlike the RAS's nut-and-bolt setup).

They're not MILSPEC 1913 rails because they're slotted for weight reduction but I've never had issues with accessories fitting. LaRue QDs may not like them. Haven't tried.
They've both been discontinued for years but can be found if you keep your eyes peeled. I believe the M77 is less common than the M63.

I usually am on top of stuff like rails, and until I saw it in person, I had no idea it even existed. My buddy traded me a UMP stock and hinge pin for it, pretty happy with it so far.