Don't worry about the willow bark vs. aspirin part. Aspirin has only been shown to increase the effectiveness in obese folks.
And the stack basically increases fat motility (lipids in the bloodstream), and speeds up the metabolism. Basically, this will let you work out harder, and burn a greater percentage of fat when you do.
I haven't had any bad effects, except excessive sweating when I take Lipokinetix (which is a totally different animal).