The Star Tribune has nearly 600 actual challenged ballots available for your perusal. I don't know whether this is a truly random sampling or not, although it appears to be. The site is a little slow and you'll have to sign into their website to be able to peruse all of them, but it's apparent that:
1) This gets old in a hurry, and fatigue is going to be something of a factor when it comes time for the Canvassing Board to review what could wind up being as many as 7,000 challenged ballots;
2) The vast majority of challenges on both sides are frivolous, often utterly so. Perhaps 1 in 10 challenges -- maybe slightly more than that -- actually required a judgment call of some kind.
It should be remembered, however, that even a small systematic edge for one or another campaign in the challenge process could wind up being decisive. With this many challenged ballots in play, if, say, 13% of the Franken campaign's challenges are upheld as compared to 11% of the Coleman campaign's challenges, that could make a rather large difference.
Furthermore, irrespective of the frivolity of one or another campaign's challenges, different types of challenges have different types of effects on the state's quasi-official count; absent more information about the nature of the challenges, it's very difficult to diagnose where we stand.
For now, however, the various iterations if the statistical process that we applied before now show Norm Coleman as the slight favorite, although his edge is small and might reverse itself if Franken is able to get discarded absentee ballots reconsidered. Look for a more comprehensive update on Minnesota tomorrow.