For a NAT connection they're a bit useless as there's no uPnP and on the version of firmware I have you can only forward 20 ports.
They aren't too bad to configure, though aren't desperately newbie friendly
TelNet open to the world is INCREDIBLY annoying as there's a server on a port with a completely blank password which can be used to reset the config to default

You can block off web/ftp though thankfully.
Unstable - mine has been fine, but a friend of mine had a different router with the same chipset and it was fine for a while but then went a bit screwy and unstable requiring a reboot a day.
Never had a problem holding up an ADSL connection here on a 50dB+ attenuation line @2272kbps.
The biggest problem I've had with mine is the auto-firmware update "feature" which seemed to flash the firmware with nothing when it couldn't find a file to update with which involved an RMA
This one has been stable for 32days+ now, only reason it was rebooted was to try the BT speed checker I think.