Things worked OK for me with version 5, but shortly after I installed version 6.0 I started getting error messages (and a refusal to sync) whenever I tried to sync iCal.

On the Mark/Space support forum, I saw some messages from a Mark/Space support person suggesting that the problem was because version 6 did some stricter error-checking than version 5 did -- but I don't believe this, because the error messages persisted even after I reverted to version 5. The only way I could get my calendar to sync was by reverting to an older version of my calendar :-(

Since Versiontracker's description of 6.0.1b4 doesn't say they've fixed this problem, I'm not going to install it.