Having beat Nethack five times I can't say it's not a great game. That said, there are a lot better games, and even many better roguelikes. Dungeon Crawl Stone Soup is my recommendation. It's insanely difficult, but in a better way than Nethack (you get killed when you make tactical mistakes, not when you trip down stairways, get crushed by drawbridges, run into various insta-kill monsters, etc). Not that I want to discourage anyone from trying Nethack... it's just not the best introduction to the roguelike genre.