I have been a dual batch 5 gallon guy. Now I don't brew as often, so I'm venturing more into the larger volumes. I can still only boil and ferment in 5 gallon sizes, so I'm making smaller beers but in larger volumes.
In fact, this weekend, I'm making 8 gallons of brown ale, meant to come out around 5%. By maxing out my mash tun at 15#, using a little corn sugar, and diluting post-boil (pre-fermentation) to 2 x four gallon batches in five gallon carboys, I think it will work nicely.