mjl: it's not user-software, it's server software for experienced administrators. To give you an idea of what we mean by this:
There is very little documentation so you'll need to be prepared to spend time searching through the forums for answers; depending on which thread you read you may end up using one of many different solutions, so you'll need to be prepared to exercise your own judgement as to which you choose.
Many of these solutions demand that you patch core PHP files, but as those core files evolve, budget time and find tools to support the roll-forward patching when a new zip file is released. Some people likely run WPMU off SVN head but there is no consolidated document stating how to run it from a "known stable" tag. Also, you should know that (as at least of last year) the code repository was not directly open for contribution so any patches you may come up with must be lodged into trac, from which a couple of people have commit access. It can be a while before patches are incorporated (in some cases 6 months+).
The plugins are scattered across the internet, though there is a partial list on wpmudev.org which is improving.
Assuming you are a competent PHP programmer and systems administrator you should be okay.
If the tasks above appear overly onerous then you should avoid WPMU, or hire someone to look after it for you.
I hope that helps,
Martin