We are looking to replace 2 Adtran 550's due to end of life. What is everyone using in its place?
Just so you know: ADTRAN doesn't have a replacement for the ATLAS products. It is simply a discontinued product line.
Keep them. They're bulletproof. You may eventually need to replace the NVRAM/clock chip especially if they've been powered off for a while. There are details in the forums about how to do this. Other than that they'll last forever. Consider picking up a spare from EBay if you're worried about failure.