I've driven my Spark on some steep hills such as in Poughkeepsie, NY, and had no problems getting around. The car does take some time to get up to speed, but once you get going, there's no problem.
When I test drove the Spark I drove both a LS Automatic and a 1LT Manual. I found the manual to be geared tall in that you have to downshift frequently. I was going 30MPH in 4th gear and the engine simply bogged down. For the auto, as long as you have some momentum, it will go up a hill fine.