This would make a fine mash tun and at $19.97, it's a great deal. I will say that I personally started out with a 5 gallon cooler as a mash tun and found it a bit on the small side. It will work for most things though. If you're into higher gravity beers I would consider a 10 gallon cooler.
Get a weldless kit for an HLT and/or a weldless kit and a tube screen for a mash tun and you're up and going.
Rubbermaid Home Depot 5 gal. Orange Water Cooler - $19.97 In Store Pickup
If you're looking for a larger mash tun, consider these...
Igloo 10 Gallon Seat Top Beverage Dispenser - $47.86 Shipped
Coleman 70-Quart Xtreme Cooler (Blue) - $39.88 Shipped