Lee Collins here. I landed a contract with them through the Open Door month for my novel The Dead of Winter. My waiting period was from March through October, and I was one of the first two announced, so (as my reader Amanda Rutter put it) "set yourself in for a wait" if you decide to submit. It look as though AR Prime is doing away with the intermediary readers this time around, though, which may mean shorter consideration periods.
One piece of advice: as Anne Lyle mentioned in the first AR thread, have at least a synopsis of a second book (sequel or not) ready to go if your full manuscript gets escalated to Marc and Lee. Before they sent me an offer for my book, they asked what else I had in mind, either for the characters from The Dead of Winter or a separate book. I whipped up a two-page synopsis for the sequel that had been rolling around in my head for a year or so, but I would have saved myself a good deal of stress if I'd already had it ready to go.
The experience itself has been fantastic. Marc, Lee, and Darren are great to work with, both personable and professional in their correspondence. I have very little experience in or knowledge of the publishing world, and they've been very helpful in teaching me the basics of marketing and conventions and whatnot.
If you have something that meets their criteria, I definitely encourage you to submit to one or both.