Originally Posted by Got Trub?
Nope. No flaws. I have both a 7g and a 10g cooler for mashing in. The 7g can - barely - mash a 1.045 OG beer with 80% efficiency and 1.2q:lb ratio. No way you could do an all grain in a 5g tun.
Like any question other than what color is green grass, you're gonna have differing opinions, & differing experiences from one individual to another. As you'll see below I've had a much different experience using a 5 gal. tun than Got Trub has with a 7 gal tun. I easily get 1.065+ OG biers with approx. 12 lbs of grist in my 5 gal tun with efficiency of 80-85%. In order to get a 1.045 OG bier I use less than 9 lbs of grist for an average recipe which doesn't come close to filling up a 5 gal tun.
I've got a 5 gallon cooler and am very happy with it for 5.5 gallon batches. Since investing in a simple corona style mill I now routinely get 82-85% efficiency. My last batch linked here: http://hopville.com/recipe/199372/other-smoked-beer-recipes/illudium-pu-36-smoked-porter
contained 10lbs 5 oz. grist which produced 1.057 OG.
My brew prior to this one was a Strong Scotch Ale linked here: http://hopville.com/recipe/188576/strong-scotch-ale-recipes/1314-the-year-of-scottish-freedom
which I got 86% efficiency on and a 1.089 OG on a 4.5 gallon batch using almost 13 lbs of grist which fit in my 5 gal tun with slightly thicker mash than usual.
I can achieve 1.100 on 4 gallon batches with my 5 gallon mash-tun. I have a Makita cooler which was listed as a 5 gallon cooler but I believe is more like 5.5 or 5.75 which helps me in terms of achieving good gravity numbers with a smaller mash tun. The down side of having a larger cooler is the heat loss issues you'll have when doing a small batch like a 1.035 Ordinary Bitter, Mild, Specialty Belgian, etc. It's very hard without a false top (piece of styrofoam placed on top of the mash) to get a good mash on 6 lbs of grist in a 10 gallon cooler.
My conclusion is it depends on what your expectations are & your brewing style. I just don't drink enough bier to make 10 gallon batches (I even have 3 kegs set aside waiting for the day I go bigger). Thus for me the idea of doing 4-4.5 gallon batches when I produce RIS, Barleywine, Scotch Strong, etc. is no big deal. About half of my brews are 1.045 or less session biers so the 5 Gal MLT works great and I would find more frustration in a larger MLT rather than the minor inconvenience of producing 'less' bier on the bigger styles. If it's a big deal, get a 5 & 10 gal MLT! I know even with a 10 gal I'll never say bye to the 5 gal tun!