Actually its fine for streaming HD movies too. I have my home server set up from a Pi exactly like this and it is fine. I have mine streaming video via mediatomb and I have never had a hiccup once, even with 1080p video.

Sure maybe transferring files to/from it is not the fastest but I find it perfectly acceptable (say a minute or so to transfer a 300MB file) and as the OP says I don't feel bad about leaving it running 24/7.

Printer serving is one thing I have not been able to transfer from my old server, seems like the Pi is not that well supported for printer drivers, but I do expect that will be resolved soon by people smarter than me.