I purchased a used Prius a year and a half ago, and I’ve been happy with the purchase so far. I haven’t had to do any major repairs yet, so even in that sense the decision so far seems to be a decent economic one.
As far as MPG, I think that the foreign-labelled cars in general try to adhere to the letter of the law more than domestics to help avoid frivolous lawsuits. The MPG that I get is pretty close to advertised, and in my experience does depend on a couple of factors:
Don’t drive it like a race car. It’s not.
MPG goes down during the winter. I suspect that this is due to a combination of the battery losing some efficiency with the colder temperatures, and the gasoline mix in Michigan to avoid freezing may also decrease MPG a bit.
The best MPG seems to be around 50MPH, so if I am driving on flat, back roads with very little stopping, I can achieve better than the advertised MPG - over 50, in fact.
Otherwise, on average I seem to get around 43MPG in winter and 47MPG in summer.
For me, the decision wasn’t just about the MPG, although that’s nice - I got what seemed to be a decent price on a used car with 23000 miles on it, and Toyotas have a good reputation for going a while without parts falling off or otherwise failing.