I've previously written a little bit about the Raspberry Pi, but mostly about specific things. And when I hear people talking about the RPI, it's mostly about specific things. So, what about the whole picture? Is it worth getting a RPI?

Yes. Go get one if you don't already have one.

Some uses:
Music player
Video player / handle a display (Synergy is available)
Lightweight HTPC
Shell server
Hardware hacking / Prototyping
Web server
Cheap NAS
BitTorrent server
OwnCloud server
Security camera

You're getting a cheap all-in-one computer with low power consumption and good connectivity capable of running more or less any simple service. If you're into hardware hacking you got GPIO pins and quite a lot of community support and sample code to get started. I recently got a RGB LCD screen (warning: a lot of soldering required :3!).

Raspberry Pi is overall a good platform for a lot of purposes, which becomes its true strength. Because it often falls short compared to other platforms. You can run a web server with OwnCloud, but it will be slow. You can share your old external hard drive and use it as a NAS, but don't expect any rapid transfers. There are a few games that will run, but the experience is better elsewhere. It can play music but the audio isn't great and might feature some static.

I can't complain on the video, though. It can play 1080p with audio via HDMI as long as the bitrate isn't too high, which is amazing. 720p works flawlessly. There are images optimized for HTPC usage available. But then again, you want to play h.264. I haven't tried any other codec, but unless there is hardware acceleration available, it will be sluggish.

The bottom line is that the Raspberry Pi is slow. The ARM processor isn't very strong, especially with graphics. The included python games feature delay and feel non-responsive, and web browsing is impossible using an ordinary browser. The included lightweight browser makes a good job, but it still isn't good enough. You will be disappointed with the performance if you have any expectations at all.

For the price, and seeing how well-balanced it is, it's sure worth getting one. There are a lot of uses for a silent all-in-one computer. Sometimes a lot of horsepower isn't required, and this is where the RPI shines.