Safe mode pointer? (for hosting)

I'd like to enable our users to have a scripting language in their hosted web pages. ASP doesn't cut it because there's really no granular control over commands and things (unless I missed something). The architecture needs to support 1m+ users (forget the storage part, that's under control). The question is, is safe mode meant for this kind of thing? Do lots of other people do this? Are there major perf implications, and are there any limits that can be placed on scripts to lessen that? Thanks for any pointers...