We are running 3 mini's with great success.
We have 2 with dual drive config's and one standard mini as our test environment
They have been running since March, without issue.
Our 2 production servers host 24 and 70 databases.
The only drawback for us, single sign-on doesn't work.
Small price to pay, to avoid constant re-booting of windows servers.
We are running FMS 10 and 11 on minis both with > 10 users, no problem.
FM's preformance depends mainly on your harddisc speed. So we prefer an external mirrored harddrive to hold the data.