I found myself playing this for hours every day
When I first got the game I was determined to complete the campaign before moving on to the multiplayer. I started out by playing solo on the normal difficulty and had a great play through! The game had a good vibe to it that makes you want to keep playing it, which is hard to find in a lot of games. As you progress through it you will see many random things like easter eggs for example and skulls which you can pick up and activate at the campaign menu. After completing it on my own, me and a few friends decided that we would play through it co-op on the legendary setting. I found this even better than playing it on my own and made me wait even longer to get onto the multiplayer!
The multiplayer has to be one of the best things to happen in the history of games. At first it starts out as a bit of fun then as you get better at it, it becomes a lot more competitive! It has good maps that you can tell had a lot of thought put into them. The pace of the game varies depending on what playlist you are in. For example in team snipers you will often find people camping in places and waiting around for their opponents and in team swat you will see people jumping around the map shooting each other in the faces.
Halo 3 also has a forge mode where you can edit the maps, change weapon spawns and so on! The amount of fun you can have with this game is endless, i highly recommend this game even if it is a bit out dated now.