I drive a mix of city and highway. I can get 46 to 47mpg on strict highway driving (70mph+), and have averaged 58mpg in strictly city (stop and creep) driving for hours.
I think it would be unfortunate to only get the vehicle do to its high mileage however. The CVT transmission, its ultra tight turning radius, roomy interior, impressive torque, ultra low emissions, and general quality should also be considered. My last 3 cars were Lexus's so my standards were not low, and I'm continually impressed with this little beast.
If you only consider the mileage, you'll find it hard to justify the extra cost unless you drive an excessive amount of miles. I leased it because I too was concerned about the battery technology, but with their recent upsurge in popularity, I'll probably end up purchasing it at the end of the 3 year lease. I hear you cannot find one on the lot anymore and waiting lists are beginning to appear here in southern CA.