In a press release issued today, Steve Jobs said, "...this is like nothing we've ever seen before. The revolutionary App Store offers iPhone and iPod touch users an experience unlike anything else available on other mobile devices, and we see no signs of the competition catching up anytime soon."
The App Store broke the billion-download barrier in April and hit 2 billion in September. With all the new iPhones and iPod touches (estimated) sold in the holiday quarter, this number should only climb higher.
Part of the store's success is perfection of the impulse purchase. It's ridiculously easy to find an app while you're in line at the bank for instance, do a bit of poking around and hit "buy!" Why not, it's only two bucks!
Competitors have been launched, some more successful than others, but Steve's right in saying that nothing else has matched the App Store's success. Congratulations to Apple!