Yes, I've been in one up to 70MPH for short bursts (turbocharged). Usual cruising speed outside of the city being around 45MPH. It's hard for me to think in terms of fuel economy in Japan, but when I lived there gasoline was killer expensive with at least half of the cost in taxes, so these cars were, and continue to be, very popular gas sippers. Civics and Corollas are considered "mid size" since the Kei market is so huge (around half of the market). The WagonR has been the best selling Kei car for at least the last 5 years, and has been around since the early 1990's near the top of sales, long before the boxy Scion Xb (Toyota bbX) hit our shores. A frumpy (in my opinion), rounded off version of the WagonR was sold in Europe as the "Agila". Now a redesign of the Agila (? I think its a WagonR variant ?) called the "Splash" is on sale in the UK, but I still prefer the boxy, utilitarian WagonR.
Some Gaijin in Japan with a WagonR video I found, pretty funny:
"Oh yeah, she's a beast"
( This is the reality for most, but people do all sorts of aftermarket stuff to them to improve acceleration, etc. - double click on the YouTube video after its done and see the related videos in the sidebar )