I share a monitor between the Mini and PC via a KVM switch. One like this should do the trick. A lot of Mini owners are doing the same thing.
Some Mini owners have reported that the display looks “dim” with the Mini (the culprit seems to be the VGA adapter to connect to VGA monitors). I think I’ve noticed that, but it is so easy to fix with my particular LCD monitor (just hit the “auto contrast” button) so it is not an issue. An FYI if you want to use the Mini on a VGA monitor.
As for its usefulness, if you’ve already got a more powerful Mac, there might not be any point, but I don’t blame you for wanting it. It saves so much desk space. But hey, if you don’t have a desktop Mac (only a laptop), then there’s your justification in getting a Mini! Everyone knows that you should have a desktop computer too! 
I get along pretty well with the 512 megs of RAM upgrade as long as I don’t load up every power-hungry app at the same time. (I had a habit with my old G4 to have Photoshop, Dreamweaver, Word, Acrobat, iTunes, Mail, several browsers, perhaps Garageband or InDesign, yadda yadda open at the same time, but since the old G4 had 1 GB of RAM, it handled it. The Mini doesn’t like to do that.) If I keep it to just Dreamweaver, Photoshop, several browsers, iTunes, Mail and perhaps one or two small programs, the Mini is fine. But of course I really want to get 1 GB of RAM, and will be doing that eventually.
The Mini’s slow hard drive is an issue when working with large (we’re talking several hundred megabytes) files (my experience is with Photoshop files). Under or around 100 MB doesn’t seem so bad, though. iTunes runs like a dream (importing an album is much faster than my PC with a similar CPU speed and a GB of RAM). Can’t say how it compares to every other Mac or PC, though it is definitely faster than my old G4, which is all that counts with me. Other Mini owners have given glowing reports of its speed and niftiness. But, it sure isn’t a high-end Mac, but it never pretended to be.
I had my Mini BTO (built to order) from the Apple store—1.25 GHz, 80 GB hard drive, and 512 MB of RAM. It cost $625 plus tax. Not a bad deal.
I think you should get it as a “desktop” Mac. It’s not a powerhouse, but for most things it will suit you fine.