I haven't ordered one but I am thinking about getting one to use as a file server. I don't know what OS I would use on it, I would like to use Debian but it looks like it is a work in progress port to ARM. A good idea I got from another forum was to use a powered USB hub to provide both power for the Pi and attached devices by connecting a USB cable to the power connector and the hub then connecting the hub cable to the Pi USB ports. I don't think I would need a keyboard or mouse as it will pretty much be a headless server so I might use something like VNC to control it.
I would like to use an ethernet cable to connect to the internet but that might not be possible as it would need to go either downstairs on the TV stand or in the spare bedroom with the CCTV DVR, but for that I would need to buy a switch and I don't know it would be worth buying one. A USB dongle might be a better option, especially as I would have 4 USB ports through the hub.
I wouldn't go crazy on a massive SD card as from what I have found out you only need to store the boot folder on the card, the rest can go on an external hard drive.
I've got a Pi but not done much with it yet. However a couple of notes.
It potentially needs more power than a USB socket on a pc/hub can provide so you might want to get a decent power supply (I'm using a spare phone charger that's designed for USB based phones but the PSU is 5v,1A)
Also not all wireless cards will work with Linux, there's a list on the RPi wiki. I've got one of the cheap ones from eBay and it seems to work.
Currently I think the only real Distro that works is Debian. I'm thinking about doing a Linux from Scratch type build but I've not got that far yet.
One of my plans is to make a media player for a non-smart TV. The other plan is to do some sort of car PC setup (GPS, Music, and a few other more specialist things)
Mine's got OpenELEC on it at the moment. Haven't really done much with it. I'll probably put Debian back on it at some point (had it on there before OpenELEC) or I might get another SD card and have both by swapping the cards around.
I keep meaning to set mine up. It's going to be all boring and all me to scan to email from my scanner (since the scanner doesn't support 'modern things' like SSL SMTP...)
I'll be using the Debian image probably because that's what I've always ran on my servers and it'll be familiar in that sense. Another idea was to hook it up with a webcam and record timelapse videos with it, but the camera is currently broken so that's on hold!