I used to be (administered a fairly active, but not huge, installation on a campus in the early to mid 90s) a big proponent of qmail for a general mail processing solution, even writing a delayed mail notifier for it (which I no longer recommend using in the general case due to the spamminess of the entire concept of DSN). Since that time, the network landscape has changed, and the default delivery method that qmail uses could be abusive.

For a specific end solution, where you don't use it as an outgoing delivery agent, I can still see it being a very good solution. However, tirwhan makes a very good point about resource usage. If you are aware of this, and deal with the local and remote resource piggyness that qmail can exhibit, it can be a good solution.