I've heard some say it's great, others say it's buggy! I downloaded the dev version of Windows 8 but I'm not sure how to run it without messing up Windows 7! Any thoughts?

As the only available versions are Dev then it's difficult to say how it's all going to turn out.
Sufficed to say, so far, it's very tuned for touch and seems to be a continuation of the themes in WinPho rather than the desktop varieties.
We'll have to wait and see what Redmond release for actual manufacture.

Important: If you decide to go back to your previous operating system, you'll need to reinstall it from the recovery or installation media that came with your PC, which is typically DVD media. If you don't have recovery media, you might be able to create it from a recovery partition on your PC using software provided by your PC manufacturer. Check the support section of your PC manufacturer's website for more information. After you install Windows 8, you won't be able to use the recovery partition on your PC to go back to your previous version of Windows.