I've had Hilton offers ranging from decent to fantastic (up to 25%), though I just happened to not be able to use them. So yes, it's fantastic stacking opportunity, with caveats. The biggest one being: you'll get your rewards from C1 Shopping as credit you can use to buy a gift card, and your mileage may vary on what your GC options are. I was able to use mine for a stay at another hotel for a relatively overpriced rate, but one I was still happy with.