Metal Slug is like a gaming cult/religion. And if you are a follower, this is a definite must have.
Metal Slug is not the original side-scrolling shooter, but it is the one that set the bar. No side-scrolling shooter has ever been mimicked, copied and cloned, more than the original Metal Slug Series.
I rank it a 10.
(Sure the graphics are sprite based, but that is part of the charm.)