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Two strokes vary wildy depending on the specifics of the design. Some are very crude and don't make huge power or get great mileage (but can still be huge fun), some are very fast, but still use a ton of fuel and others are very nearly on-par with current four strokes.
I have a few two stroke MX race bikes. They are astounding from a performance and reliability standpoint. They make huge power in a light package and seem essentially indestructible and if something does go rebuliding the whole engine takes no more time than a valve adjustment did on my CRF. These are liquid cooled, reed valve and exhaust valve engines with huge carbs and aggressive porting. I've never run one at constant speed on the highway and doubt I ever will. Fairly casual trail riding (which still includes lots of wheelies, powerslides etc) will get about 70 miles from a 3 gallon tank. Street use would be better I think for mpgs.
I also have a kawasaki H1 with expansion chambers. Its a 500cc air cooled piston port two stroke triple. Kawasaki really only had two design goals in mind. First, make the fastest production bike of the era and second be able to sell it for <$1k. They succeeded in both. Its crude, vibrates, uses a bunch of fuel (about 18mpg), smokes and has a narrow power band, but its still huge fun!
Generally air cooled piston port two strokes are pretty inefficient and have narrow power curves. Some manage to do pretty well though, for example Tomos mopeds are pretty efficient and not far off four strokers.
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