The last time I tried Darrell's import script (has been a real long time) it created the user_id's randomly, rather than incrementally... like the standard SME process. After adding nearly 2000 users, the system just took forever to do much of anything. It seemed to periodically lock up (had to be rebooted) stuff like that.
After wiping the system and installing using this
http://tech-geeks.org/contrib/loveless/batch_users/ everything seemed to work much better.
However, I did notice that it will only add about 500 users at a time, before a reboot was necessary. I'm not sure why, but when I did it, I would add 400 then reboot, add 400 more, reboot... repeat until finished.
I have nearly 2500 users using the POP3/SMTP services of an SME 5.5 system, and ClamAV.