From the friends that had them they loved it, small and quiet and perfect for cramped quarters though upgrading is rather limited but for the everyday person you wouldnt need to worry about upgrades. You can get used ones for around $250 - $400 depending on wich generation wether its on ebay or craigs.
Check out the refurb'd listings at the Apple Store to catch a break on a "new" Mac Mini. I picked up a 1.83 GHz Core 2 Duo for $499. 4GB can be had for under $100 if you need more than the Gig that it comes with. If you want something compact with Windows, Dell's new Hybrid or the ASUS Eee Box would fit the bill.
I think the Mac Mini is a great little box. I use mine to Fold proteins and watch iTunes content on my 40" Toshiba LCD via HDMI.