The N64 might just seem like an old, outdated system these days, but I would disagree. This is actually the system of choice in my college dorm. On the N64 box, it says "The fun machine, " and I think that's why people still play it: it's pure fun. The graphics may be horrible by todays standards, but that's alright; the gameplay makes up for it. Some of the best multiplayer games ever made were made for the N64. Games like Goldeneye, Mario Kart, and Super Smash Brothers were born on the N64, and they still keep me and my college buddies up for late night gaming sessions.
I guess I like the fact that the games are a little simpler than many games out today. The controller doesn't have as many buttons as most consoles do today, and thus many games can be picked up and played fairly easily (of course, mastering them is a challenge). There's just something about blowing up your blocky, square faced friend in a game of Goldeneye that is still just as fun today as it was 10 years ago. The focus is more on the fun of the game than the prettiness of the graphics.
Today, the N64 can be found for really cheap. Don't just go out and buy the newest console out there. Yes, the graphics and sound aren't up to todays standards. But give the Nintendo 64 a chance, especially if you're into multiplayer games with your friends. Old school games are still great fun.