I have been using the Coleman xtreme 70 qt cooler as a mash tun for about a year and a half now. I love it. I have the ability to make huge abv 5 gallon batches with ease and can probably do 10 gallon batches as well. I do have to say though that I have not done a 10 gallon batch yet though, so I can't give you a full proof answer on that. I have had 26 lbs of grain and mash water in there at a time though.
There is a fair amount of headspace when doing low abv 5 gallon batches. A lot of people think that it won't hold the heat well enough, because of that, but that's bologna. Throw some tin foil over that mash. I barely lose more than a degree when mashing for an hour.
I love my Coleman xtreme.