I’ll be honest, having used lots of devices without and a few with, I am firmly in the “neat, but so what”-camp.
And I would surmise that this is overall how other companies see this: They could compete, but there’s just no reason to. Samsung doesn’t sell their phones and tablets because they have S-pens, they sell them because they’re the Apple of Android phones, they sell brand name for a sizable markup. That’s a soft goal to compete with, and hence not a good one to go after with one specific hardware feature.
Unless there’s some incentive, say a new mode of UI interaction that is entirely based on stylus interactions, I can’t see other manufacturers being interested in it at all.