When I use Live Mesh on my Mac, it often pegs the CPU (up to 160% cpu usage!).
I have also found some unreliability with Live Desktop, where documents that have synced no longer appear on my Live Desktop. However, those files continue to sync from my Mac to the desktop and from there to other devices. I just can't see them on the web.
I deleted all my preferences on the Mac side and re-joined the Mesh and I appear to be back to normal on that front. I am still seeing excessive CPU usage, however. (Especially when compared to DropBox or SugarSync, neither of which takes much CPU time at all.)