MechaPilot
Explorer
See, I don't care for "momentum" stuff because it tends to involve a lot of bookkeeping/tracking hits and stuff like that. It's not the sort of complexity I enjoy. But that's just me.
I've just started work on some major NPCs for my setting. One of them, uses a momentum system for his special abilities. When he hits he gains one momentum die. If the hit is a critical hit, he gets two. He can have a maximum of three momentum dice. If my PCs run into him, I plan to track his momentum by just placing 1, 2 or 3 dice near where I'm already tracking his HP and other info.