Aside from a few little pieces, the SPA-3102 is a great unit for the $90 price. The firmware is very complete in terms of tweaking just about every setting, and each port (FXO, FXS, SIP) can be treated as a unique entity, which makes it perfect for this purpose. I did upgrade to the latest firmware (5.1.5GWa as of this writing) without issue, though it requires a Windows system for the upgrade. The Web interface is a little wonky at times, but is otherwise fast and responsive. I especially like the fail-open configuration -- if power is out, the FXS port is bridged to the FXO port so normal calling is still possible.
I'm expecting another one of these for another PSTN line, and possibly a Digium Iaxy to play with. Suffice it to say, I'm getting into Asterisk in a big way.