From my anecdotal "research" on two very different TW's (one used almost exclusively off road, another used mostly on-road with some light trail use). Keep in mind these numbers are all "finger in the breeze" quotes, don't go nuts scrutinizing them.
The stock 14/50 gearing is pretty good if your use is an even mix of around-town road use, rarely/never use highways, and off road regularly with some tighter singletrack. MPG will hover around 60-70, and your realistic max speed is maybe 65, but it's happiest cruising not much past 55.
The popular 14/47 gearing is good if you tend to favor the road a bit more over trails, or when you do ride off road it's two-track or easier/faster singletrack. This can still handle tight stuff and hills OK, but in the gnarly stuff you're going to be on the clutch more. I'd approximate you get about a 5mpg boost on the road, and cruise speed comes up to the low 60's.
On my ATV tire equipped mud bogger TW, I ran 13/50. This also had a mod engine with stage 1 cam, piston, exhaust, jetting. Top speed was down to about 60 (those heavy chunky tires really sap power), and MPG was 50 at best. Then again, I was using it probably 80% off road/ 20% street, so road speed was not a big concern. You could stick in in first, floor it, and sink it in mud to the seat and it'd still keep that 8" wide Duro churning away no problem. The first time I did that (still on the stock chain), it snapped clean in half. Coupled with the extra snot from the engine(and a quality EK chain), 2nd was useable for long/softer hill climbs where you'd need some speed and couldn't crawl it in first. It'd be able to hold high rpms and keep the rear spinning up a sandy hill without bogging out and needing a frantic mid-climb downshift. This was in NY, so woodland trails that were often muddy, and there were a lot of quarry sand pits we'd play around in. 13/50 is quite low- 1st is a total crawler gear at this point and completely useless on the road- I'd always set off in 2nd, and still shift to 3rd almost immediately.
On my second street-oriented TW (stock rear tire), it came to me with 14/47 and a slip on/jetting. This seemed pretty perfectly matched to the engine's power. Once I did the stage 1 cam/piston swap, it felt a little under-geared. Acceleration was great, but I'd bang off 4 quick shifts and then leave it in 5th pretty much unless I had to stop again. Ended up going to a 15/47, which is better matched and keeps RPMs down even further on the road, since the engine has extra power to pull it now. I won't say 70mph is "relaxed", but it doesn't feel painful to the motor anymore, and top speed is into the 80s. This is in Texas, so much different riding climate. Mud is essentially unheard of, and it's much more hard/open/rocky so I don't need massive chunky tires and super low gearing to turn them.
