They work well. I really liked mine for daily driving, it allowed for a good low end feel that carried well. Instead of being stuck with narrow power band. I didn't have the cranky idle knock as bad as some of the bed I've been around.
Oh… and on cold mornings the truck started as if it was summer.
From my understanding of it, no experience yet but want one, you can have the timing set lower for low end and the spacer will still bump it up for the top end, best of both worlds kind of thing. I want one to try and gain some low end.