Fantastic 2D platformer that I will never quite get tired of, I still play it every now and then despite it being an old purchase.
I first heard about this game by watching a Let's Player play the xbox version, I was immediately convinced to try the original PC version (as the updated one wasn't out on PC yet) and loved it. If you wish to try the old PC version before purchasing this one, by all means: http://spelunkyworld.com/original.html
As for the updated version, a lot of good new features and items, still very difficult and still very fun. A real test of your reflexes and mechanical skill as it often pits you up against one-sided situations that must be bested before you can claim the title of having beaten this game. The difficulty is also very fair, so you get to blame yourself for every death.
The game has fifteen default levels (and a boss at the end) that must be beaten in succession, after learning the game in and out you'll be surprised to hear it's not over yet as there are another three levels (and a boss at the end!) after the "final boss". There are also five hidden levels. Oh and you must beat all of this in succession, in one single run, in one life.
Has a score attack element to it too, when you eventually get good enough to consistently beat the game you can try to get the highest score, still plenty of fun and lots of challenge.
It doesn't matter how many times you die, you will want to try again and again and again to get further and further than you've ever been before! All levels are randomly generated, so you will never be the same place twice.