ReactOS is an open source operating system that has been in development for 10 years. The main goal of the project is to build an operating system that is completly compatible with Windows XP. It uses all free software and uses a similar directory structure and program names to Windows. For example on the desktop you can see on the desktop a command prompt instead of a terminal or shell. Also people will notice a similar start menu and small things like notepad. What this also means is you can use binary exe's on ReactOS. One of the biggest differences people encounter when moving from Windows to a *nix system are that there isnt a universal installer and there are several different filetypes and methods of installing software. I would deffinitely recommend downloading firefox instead of using the ReactOS ibrowser which was not very good.
I think this is a cool project, but development seems to be going slow as I mentioned before it is in its 10th year and so far the newest version is only 0.3. It would be cool to see this project gather some speed although it cant keep up to corporations like microsoft or other opensource operating systems like Linux which has a much larger community. So it is a nice project and concept, but is not quite ready to use as a standalone operating system. I hope to see it develop faster in the near future as it has recently had some press on social bookmarking sites like digg.com.
As for speed compared to Windows or Linux distros I cant really say because I tested it in a virtual machine. They offer a vmware image on the site so anyone who has the free vmplayer can try it out. This is one of many download options they have on the site.
The features I thought they did a good job of migrating from windows were the filestructure, the explorer interface, the start menu, the task manager, the registry editor, and the obvious one is the exe's which I mentioned before. I believe but I am not sure that they are using WINE (Wine is not an emulator) to accomplish some of this which im not sure is such a good thing. I havent really tested too many things but in my experience with wine on Linux it doesnt work most of the time it might be different with ReactOS because it is closer to Windows. I will test it out with some apps I had trouble with wine on Linux and come back to this.