I've been using ISPconfig for some time on my production servers, with no problem what so ever (except when I did something wrong).
On one of my server I have 72 websites, and just over 400 email accounts, and ISPconfig has no problem handling this.
I know that some other "big" users here have more sites/accounts on their servers, and as far as I know also no problems.
Personally I think that in some areas ISPconfig will perform even better than commercial programs!
Never execute code written on a Friday or a Monday.