I have a 16" RRA AR15 with the two-stage trigger. I've had it for a bit over a year now, I think (I'm so bad at keeping track of years, months, etc. - it was this time of the year when I got it, at any rate). The trigger didn't so much go to a long "single stage" trigger as it went to a single stage trigger with a (just guessing here) 3 ounce trigger. It gradually required less tension as time and rounds went by, to the point where it would double-fire with a single trigger press. I should note that I did notice the trigger getting lighter, but just figured (falsely) that it was wearing in.
I sent it back to Rock River Arms early this spring, IIRC, and they replaced, tuned or did something else to it. I imagine I put somewhere close to 1,000 rounds through the rifle between the time I got it and I sent it back under warranty for repairs. Since getting it back I've shot probably another 500 rounds through it, and haven't noticed any problems - I imagine it's a somewhat 'hit and miss' in terms of failure, so Rock River only tunes/fixes the bad ones. Or maybe it's endemic to the design - I don't know. I do know that I recognized the problem before sending it in after only the first shooting session, maybe 100 rounds max.
Were I to do it again, I might forgo the Rock River 2-stage trigger, as I've also heard they tend to have this problem. However, I most certainly would get a 2-stage that's known to be reliable.
(Due to the low tolerance of part measurements in 2-stage triggers, it's natural to assume that they'll be less reliable, overall, than standard triggers. I think it's reasonable to assume the standards will last a substantial amount of time longer before needing replacement or tune-up. However, I believe that many of the 2-stage triggers out there now offer user-adjustable set screws, making the fix fairly trivial. Don't quote me on that, iv'e got no 1st person experience in that dept!)
For a description of the trigger differences:
Single-stage triggers, at least on ARs (and in my experience) are what you'd expect on a combat weapon: not tuned in the least bit, and fairly long squeeze, with a trigger weight/tension substantially more than you'd expect to find on a good hunting rifle (or, at least, as my hunting rifle, a Tikka T3, has). There is no "glass rod break", as they say - though I have heard that it is fairly easy for a competent gunsmith to lighten and keep reliable (ie don't try it at home/unless you know what you're doing).
Double stage, on the AR, is essentially the same, except instead of the consistent, long trigger pull, the first bit of let up is really, really soft - maybe 1/5th of an inch, if that. It's just enough to let you know you've started to engage the trigger - which is good, because the 2nd stage does 'break like a glass rod'. That is, it's crisp, relatively easy to break, and feels finely tuned.
I believe I've read that the Jewel 2-stage triggers are essentially the best you can get for your money - but also some of the most expensive. I dont' recall if they're drop-in or not.
Hope this helps.
