In my opinion, a timeless shooter. Classic and just made for pure fun, hours and hours can be spent just playing the game to clear the challenging levels. Probably one of the best soundtracks around till today too. Don't start talking about level balance, weapon balance, lack of story etc. just shut up, open this game and play it and you will not regret it. Hey, I've played this game over 12 years and I certainly have not regretted it.
