that's what i've been using for 5 and 10 gallon batches. It was a little too full for 10 gallon batches of higher gravity beers. And it's now fallen apart thus my quest to find an alternative. Since I already have the big Igloo, I'm hopeful that it will work for 10 gallon batches.
Incidentally, when I refer to 10 gallon batches I really mean that I have about 12 gallons in primary and rack off 5 to secondary. So my volume in the mash tun is slightly greater than if I were to have only 10 gallons in primary.
I define batch size as how much I get to drink when I'm done!