I have an '81 carbed 300 and it is a beast of a motor. However, if you're looking for top speed, you won't find it here. It's lack of horsepower just doesn't get the big Broncos moving very fast. My top speed on the flats was 70mph and that was with a tail wind, back seat removed, and the top off.
With mine, I get around 18 - 22 mpg on the highway, depending on how I feel like driving that day. My lowest mpg was 17, and that's with a completely full load of luggage, belongings, and people over a distance of 450 miles in 10 degree weather. You can't argue with that. And this is on a 25 year old motor that's never had an overhaul or rebuild.
Off road, they are as most say, unstoppable. You won't go blasting through things like the big V8s, but you'll always make it. They have a great amount of torque at very low RPMs (specs say 255 @ 1400 RPMs) and never bat an eye at heavy work.
It all depends on what you're looking for. If you want a powerful trailblaster, get a V8, but if you want an unstoppable trailcrawler, get the I6.
All in all, V8s are cool, I wouldn't mind have another Bronco with one, but I won't trade my I6 for it.