Experience with stacked subwoofers?

I have seen a handful of responses to posts mentioning stacked line array subwoofers, like the stackable Rel 510s. Does anyone have a stacked subwoofer array in their system and can speak to their experience?  The marketing hype that Rel puts out about this has me intrigued. Their explanation as to why it’s awesome makes sense, but also I want to go beyond the marketing and hear about real experiences before I drop a bunch of dough on something like that.