I'm a Linux user already. Should I use LinuxMCE's distribution
Strictly speaking LinuxMCE is a package. A very big package. The distribution it runs on is standard Kubuntu Linux.
Only the PC running as the Core is required to run Kubuntu and use the LinuxMCE package.
Any PC used as a Media Director can netboot from the Core, without the requirement for an OS at all. Of course, you can always boot whatever OS is already stored on the hard drive of each Media Director PC and use that PC separately from the LinuxMCE system. Play the games (or whatever) that are stored on the hard drive (using whichever OS is already installed there) there, and when you're ready to use that PC as a Media Director again, you just go back to netbooting from the Core.
For the Core, you should use Kubuntu and our packages.
LinuxMCE is based on the standard Kubuntu Linux distribution. While you could try to run LinuxMCE on another distribution, it's not recommended. LinuxMCE has lots of scripts and utilities to facilitate various tasks. Plus, LinuxMCE embeds many other open source projects, like Xine, Asterisk, etc., with our own "wrappers" that allows them to work together seamlessly. These wrappers are only tested against the versions we maintain in our Ubuntu mirror.
Before we upgrade anything on our mirror, we thoroughly test the new packages in our system to be sure it still works.
We didn't take any modules away from the Kubuntu distribution, we only added some. You can still use your Core as a normal Kubuntu Linux PC. In fact, our additions can be bypassed if they get in your way, as is explained below. But in general, DCERouter and other devices, like the home automation modules, all run in the background and won't interfere in screen sessions.
Here's what we changed:
LinuxMCE includes a lot of scripts that automate all aspects of maintaining a Linux system. For example, when you add a new user from the LinuxMCE Admin site, it automatically creates a new media directory, exports a samba share, creates an email and voicemail accounts, and so on. There's also scripts to do lots of low-level things like setup ip network prioritizing, so your VOIP calls are always clear. By default, LinuxMCE overwrites and re-creates many Linux config files at each boot. In this regard LinuxMCE acts like a black box appliance. However, once you install LinuxMCE you can go to the LinuxMCE Admin website-->Advanced-->Boot Scripts and disable any or all of these scripts. Then you can still maintain your LinuxMCE Core like any other Linux PC and it won't overwrite your config files.
Our Ubuntu mirror
We maintain our own mirror based on Ubuntu. The reason is that we have "wrappers" for many open source projects like Xine, Asterisk, etc., allowing them to work together seamlessly. Whenever updates are posted to Ubuntu's repository, we test the new versions to be sure the changes did not break anything in our wrappers and that the auto-configuration scripts still work. In very rare cases, we need to make our own version of a package that replaces the one in Ubuntu. We avoid that whenever possible, and instead re-work our wrappers so that we can stay current with the "stock" version. Such changes are generally trivial, and you should see no difference between LinuxMCE's Ubuntu mirror and the official one.
The Media Directors use our own modified version of the Ratpoison window manager. All applications appear as full-screen, with no borders. Our Orbiter GUI becomes the XWindows desktop. This is the desired behavior in an appliance-like set-top box; we completely isolate the user from anything computer-like. However, you will likely not want this as your main desktop. It is possible to run 2 versions of X on separate terminals: one with LinuxMCE's window manager, and the other with your own preferred choice of desktop.
See also: Overview of the software modules