I am a big Halo fan; I just like the style and presentation Bungie does in their games. The story telling is great, even is the story is a bit lacking. I love their art direction and styling, colors are vibriant but not cartoony. I say just go try it at someones house and see what you think. I'm not saying it is the best game ever, but it obviously does something good if so many people like it. Just like everyone, there are games I like and don't like; I don't understand why people love Zelda, looks like the exact same game everytime. My girlfriend has the new one for Wii and I watched her play, I got bored very quickly but there are those who love that.
Multiplayer is just one of those things you either like or don't. If you grew up playing lots of Halo 1 multiplayer with friends, either Halo 2 or 3 online will be a great extension of that experience. Unlike Jimmi, I like the matchmaking and not picking the exact map and game type each time. I like leaving it up to the system and enjoy the challenge each time with something different.
Here is my opinion; If you are more a single player style gaming, just like playing a game by yourself, then I might not suggest Halo 3. While there is a single player campaign, it is pretty short and does have some replay ability, but not enough to make it last months. Halo 3 really shines when you add in the multiplayer. You can do co-op campaign and up the difficulty to give a challenge, or go online and in multiplayer. You can also share screen shots or videos and such. If you've never played a Halo, go find someone with Halo 1 and play the campaign. That is top quality level design and presentation.
