7.0b8 working fine here as email preprocessor shielding Exchange server from the big bad Internet. Also have another working as server/gateway for a small office, no problems at all.
An upgrade from 6.01 failed, but since it was not a bone-stock 6.01 (SSMTP & CUPS installed) and the 6.01 hard drive had crashed horribly I didn't figure it was worth a bugtracker entry. Another install failed with a kernel panic but that turned out to be a dud CD drive.
I've not tried to run 7.0 on anything less than a PIII/600 with 192 megs RAM but it runs great on that.
7.0b8 looks very very strong! Major props to the dev team for what looks to be the best SME ever.