500 class (in my opinion) is perfect for a beginner or someone moving up from a scooter. However, my wife's 250 costs us about $60 a year for insurance with theft, roadside assistance, and everything, you can find them around a thousand dollars and she gets an average of 75mpg.
I feel 250s are too small, I get cramped, and the lack of power leads me to almost redline every gear getting up to speed. 500 class have enough torque to get up to speed quickly and easily, and being a bigger size my legs and back appreciated it. My first bike was a GS500 and it was PERFECT for me.
I've made the argument before that I feel the FZ6R is a great SECOND bike, but it does have many attributes that make it beginner friendly... but I won't get into all that again. If you really wanna know, email me
Depends entirely on you. For me, the FZ6R has been the perfect first bike, I'm into my second season with mine and have put around 6000km on it. Never any trouble, it's comfortable, nimble, fast, smooth and linear and easy to control and ride. I did 55mpg on a trip down to the states. I do 45-50mpg the rest of the time.
Yes like a car, driving stop n go city will kill your mileage. Yes its a VERY quiet motorcycle.