Here we have a perfect example of a good game that, with a few minor adjustments, could have been so much better. It's one of those games that feels like it should have been great, but when you've finished with it, you've still got that annoying feeling that something's missing.
I don't mean to say that HITMAN isn't any good, because it is. Once you get into it, it's a really engaging experience, darkly atmospheric and full of style. As the nameless contract killer you play, your missions are interesting and varied, and although you'll have to accomplish all goals to get through the game, the way you execute them (no pun intended of course) is usually up to you. Stealth is key, and one of the coolest features in HITMAN is the ability to don the clothes of anyone you kill, allowing you the fun experience of using disguises to get closer to your targets.
Despite being quite an interesting and unique experience, HITMAN is not without its flaws. By far the most annoying is the fact that you can't save your game. Ever. The only time your progress is saved is when you finish a level successfully, which means that if you make even one mistake at the very end of a mission, well, it's all the way back to the beginning with you. Programmers, are you listening? This is not okay. It leads to a lot of resentment and frustration, especially if you're having trouble getting used to HITMAN's awkward and complicated interface. The game tries to make up for it with an odd system of reincarnation, but even that's not very helpful, and only seems out place. The point is, you're going to need a lot of patience to enjoy this game. And you probably shouldn't keep your computer anywhere near the window.
HITMAN's plot is sketchy at best, and comes dangerously close to the whole cliched mad-scientist thing near the end, but does manage to save itself and stay interesting. The writing is quite clever at times, the music is well done, and even the voice acting isn't *too* bad. As for graphics, they're about on par with the standards at the time it was released, and although they're technically nothing special, the game's programmers did a great job turning them into some genuinely beautiful environments, such as jungles and ornate hotels. In fact, when HITMAN makes use of those massive maps and gorgeous surroundings, that's when it's at its best.
My advice? Give it a chance -- but play the sequel first. HITMAN 2: SILENT ASSASSIN has all the style of the original and almost none of the problems, and playing it first will make its predecessor a lot easier to deal with.