Just a tip that I missed when it first came out (but have been using it with suprisingly good results for the last month or two after re-reading the what's new for 2006)
When I got overdefined or unsolveable sketches and went to display/delete relations for the sketches to root out the problem, I would see all the sketch relations that are in error lit up in reds and yellows and have to use experience, instinct, and the filter for external relations (usually the culprit) to figure out what was wrong.
in 2006, SWx added a 'resolve sketch conflict' to the Tools>sketch tools menu. Kinda hard to find, but worth it.
This thing works great - it weeds out most of the red (haha) herrings and points to the few relations that are the actual sources of the issue. I can't remember the last time I so thoroughly bought into a new, automated thingy in SWx. It has worked great every time I tapped it.
Just thought an extra mention might help someone else out - I searched and saw no mention of it here Ed