Raspbian - 5/5
From the outset, Raspbian gives the user a bland, but functional desktop. Using Xfce as the desktop environment means the RPi's resources are kept well in hand, and not wasted on inefficient eye-candy.
However, Raspbian being a Debian-based distro means that the desktop can be altered to significant effect. The likes of Mate for Raspbian can be installed, giving the desktop the same look and feel as a classic Gnome 2 environment, which means you can then go forth and tweak it to your heart's content.
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In terms of usability, Raspbian flies ahead of the pack initially, but a trigger-happy user may get carried away with the apt command, install hundreds of programs and make the OS start to feel like a pig in treacle.
Arch - 4/5
Arch is a funny one. It starts life on the RPi, displaying nothing more than the Terminal; but if you take the time to delve into the depths of Arch, you'll come across one of the best operating systems there is.
Arch, by its very nature, is a streamlined OS, and even after installing a desktop environment such as OpenBox, the system responds quickly. In terms of looks, if you combine OpenBox with a Raspberry Pi theme and Conky, you're well on the way to a very pleasing desktop.
Again, as with the looks side of things, Arch's usability is all down to what you want to install, which makes Arch - eventually - the most configurable and usable OS. A few installed packages, and you'll have Arch running like a charm and running rings around its bigger, fatter brother, Raspbian.
Android - 3/5
Android 2.3 in itself is a pretty good-looking operating system. Decent animations, high-spec icons, themes, animated wallpapers and desktops make for an eye-catching environment. When used on the Raspberry Pi, though, things tend to go a little awry.
The current Android project for the RPi is far from perfect, but it's a project that will be completed, and one that will eventually outshine the others - we're just not sure when exactly.
In its current guise, working on the 256MB RPi, it's borderline unusable, but the increased RAM of the 512MB RPi can make things a tad better. It's still not something you could use day-to-day, but keep an eye on the project, as it'll soon come to the forefront of RPi news.
Risc OS - 4/5
Risc OS, like Raspbian, takes you into a nice GUI at startup. The Risc OS GUI is a well laid out, and colourful environment; it's also a nice change to have an environment boot to 1080, providing your monitor can cope with that resolution.
In terms of looks, the base desktop is probably all you'll ever need; it resembles the glorious past that Risc revels in, making it feel a little retro, but not in a poor-quality, 8-bit kind of way. In the usability stakes, Risc OS starts off a little niche, requiring some previous experience to get off the ground.
For example, the Ethernet port is disabled by default - there are full instructions on how to enable it, but doing so could put off a newcomer. However, perseverance, as with most aspects of computing, is the key and we can guarantee that within an hour of first use, you'll be navigating Risc OS like a pro.
Plan 9 - 2/5
If you've used Windows, Mac OSX, Linux or even Unix then forget everything you thought you ever knew when using Plan 9. Although the commands are familiar, getting to them is something of a task.
Unfortunately, the level of understanding here represents a rather steep learning curve, which makes it a little awkward to get off the ground; but there is plenty of documentation out there.