I had a 2007 5.7L 2WD. Yeah, the gas mileage wasn't great but it wasn't much worse than my DCLB 4WD Tacoma. (13-16mpg vs 15-18mpg)
I must say that I really do miss the beast, especially the interior room....and the bed size....and a lot of other things about it (like the sheer power and f'ing awesomeness of the truck). However, it was a bitch to park and I really wanted a 4WD again.
In the 20 years before buying the Tundra I only owned 2 trucks. An '88 4WD Reg cab Toyota Pickup (on 38's and later 36's) and a 2000 4WD Extended Cab Tacoma (baby truck on 31's). After being refused entry to a local beach access because i didn't have 4WD I decided I needed to go find another 4WD. A Tacoma seemed like a logical, sensible and loyal purchase. I've always loved Tacomas and I had recently fallen in love with the DCLB. In addition, it's a lot easier to build up; it's cheaper to lift and you don't need to buy huge tires to make it off road worthy.
My biggest fear about buying a new Tundra was the huge expense of replacement parts later in the truck's life. Remember, in a 20 year span I'd only owned 2 Toyotas. The thought of replacing a @20K motor or a $10K transmission years down the road scared the crap out of me!
However, I didn't realize that I'd have to take my Tacoma to the dealer several times over the past 8 months to fix stupid BS. I've had the blower motor replaced, rotors turned (still squeals), TSB springs installed, and the windshield still whistles at highway speeds. WTF? This part of Tacoma ownership has been disappointing to say the least.
Overall, am I happy with downsizing? Yes and no. I miss the power and size of the Tundra but the Tacoma is more practical and less flashy. I had thought about trading the Tacoma for another Tundra but I think I'm going to stick it out. I guess dropping a few grand on lift and tires over the past few months has changed my mind for a little while.