This is a great plugin that can handle pretty much any e-commerce needs. It's simple enough for basic online payments yet full featured and extendable enough for complex e-commerce solutions.
With their first class support, it doesn't get much better.
As this was the first legitimate e-commerce plugin for WordPress, and one of the first commercial plugins, it was a pioneer in many areas. They've learned a lot of lessons over the years on their path to having a great e-commerce solution for WordPress.
It's free. Download it and try it out for yourself and you'll be the next person writing a 5 star review.