Friday seemed manageable to me. There were a lot of people, but the waits were not bad. Can't answer for Wednesday and Thursday, but my guess is that they were similar; probably a little less busy. Saturday and Sunday were pretty mobbed. We actually just gave up on Saturday. By 11:00 on Sunday, the MK was packed, although it was early entry that day, so that explains it. MGM was as crowded as I've seen it on Sunday, although the waits for the major rides were still under an hour -- I understand it can get worse.
We were told no on virtually every last minute PS we tried for starting Thursday evening and ending Monday. The only thing available for any of our days that we could find was Boma (which I thought was supposed to be easy) at like 9:45 one of the nights. We were, however, able to get walk up seating every where we tried, including the Cal. Grill. Lines for transportation were pretty good, and we only had to wait for multiple monorails once. Never been there in the summer, but my hunch is that Friday, Saturday, and Sunday were somewhat consistent with your average summer weekend, with Sunday maybe being a little more crowded.