I've used ExpanDrive since it came out, and it's the most useful paid utility I have. I manage files on several different computers and some supercomputers, and being able to mount everything as a drive is incredibly useful. I keep my iTunes library outside my laptop, but I can easily mount it with ExpanDrive and play my music anywhere with just a decent connection, which is wicked cool.

MacFusion is okay, but as I use this on a laptop and move between networks a few times a day, MacFusion did not gracefully handle loss of connections while ExpanDrive has no problem. Over MacFusion, ExpanDrive also has a super slick interface and very nice AppleScript support which I use for making simple one line location commands for mounting a set of drives at a time.

If you frequently work with remote files, I have seen nothing better - it even has better reliability and behavior (if not throughput) of NFS on local connections.

A mac without Quicksilver, TextMate, and ExpanDrive is not really a mac, as far as I'm concerned.