One thing to look out for is the infamous cold start tick with the 3.7 engine. Some of Mustang owners with this engine are reporting the cold start tick issues. My wife has a '11 Edge with the 3.5 and it exhibit's this same problem since the 3.5 and 3.7 are basically the same engine. Ford is investigating the problem.
So my recommendation is to cold start one of these vehicles before purchasing and if it exhibit's this problem (sounds like marbles rattling in the engine) you can make a better informed decision on purchasing one or not.
Wife and I are looking hard at the MKX to replace her '08 Nav. She drove one yesteday and liked it ok--just not anything like a Nav. I believe we will probably get one in about 2 weeks, if we can find the color/interior she wants. One thing, if you don't need the heated/cooled seats, an Edge is pretty much the same thing and you can option it out to most of what the MKS has for about 4-5 grand less.
There has been a lot of forum conversations about problems with the nav system--both complexity and reliability. It is way above my ability to work it, but the salesman who gave us the demo ride could work it just fine.
I believe it to be a good vehicle.
I have heard some rumors that the 4 cyl ecoboost may be available in the 2012? You can get the 6 cyl eco in the MKT, but that is getting back to Nav territory. I would guess the MKT will eventually be the successor to the Nav?
I have looked at a couple of Limited Editions 2010 MKTs--pretty nice units and another 4-5 grand less, but the dash technology just doesn't compare.