I have been very favorably impressed with some GM cars I have driven as rentals in the past year. I can't say the same for either Toyota's Corolla (though the Camry is nice) or Honda's Civic, which is less refined, less powerful, less everything than anything else I have driven. The GMs got good mileage, ran well, were comfortable and ergonomically better than I had any right to expect. If I were buying a car today, it would be a Buick or a Chevy.
Are you comparing apples and oranges? What were the GM cars that you had as rentals, as compared to the small Japanese cars you've driven. You can hardly compare a Grand Am or G6, or even a Malibu, with a Civic or Corolla - they're in different categories.
Having said that, I am also unimpressed with those small Japanese cars. I find their engines buzzy, the cars noisy overall (engine and road noise), and the level of performance, ride and handling generally kind of lacking. But that is true in that class of (subcompact - compact) across the board with the exception of a few pricey exceptions. I had a Toyota Rav4 rental last year and was surprised to find it just as buzzy and gutless as my Mom's Hyundai Elantra, maybe more so, and the Elantra's price undercuts the Japanese by a good bit. The Japanese small cars do have a small advantage in fit and finish and tactile refinement (and possibly long term reliability and durability, but that's not certain) over the competition, but the difference today between a Civic or Corolla and something like a Chevy Cobalt or Dodge Neon is not all that great (they're all little crap boxes, in my opinion).
I sometimes find myself wishing that someone would put a somewhat larger, more refined engine and transmission in this class of car, at a reasonable price.
I have a new Chevy Cobalt- great car.