Score: 4 of 5--The Live Mesh install was extremely easy, and with one exception, was flawless. And I'm probably being a bit unfair about the one exception, because it had to do with SilverLight not installing properly, but I'm not dinging Live Mesh for that one.
To join the tech preview, just sign up. They are taking 10,000 users initially. You'll receive an e-mail when you are in, then go back to the site and sign in with your Windows Live user ID.
First, you'll need to add your device to the Mesh, which causes Live Mesh software to be installed on your computer. It all went very fast and went well for me... no problems.
Live Mesh Add Devices Page
From there, you can add other devices or start to upload documents you want to share through Live Mesh, as shown in this screenshot (click to enlarge).
That's the good news. Now, for the bad. To use Live Mesh you'll have to enable UAC on Vista. Yep, you heard me right. You have to ENABLE UAC! OMG, hasn't Microsoft learned their lessons about User Account Control? You know what UAC stands for, don't you? User Annoyance Center. I'm sure there are good security reasons for doing this but Microsoft is asking a lot from us if they expect users to live with UAC in order to use Live Mesh.
Holy barriers to entry, Batman. One BIG demerit, to what otherwise is a great installation experience.