I played through the Halo 3 campaign at the 'Normal' difficulty to start with. I typically play normal and sometimes even easy when playing games these days... I just don't have the time and patience anymore. But the normal difficulty was actually pretty easy, and left me wanting more. Besides, I finished the campaign in probably 8 hours, which seems really short for a $60 game. I know, I know... multiplayer. But still.
Now I had heard that with Halo 3, legendary is even harder than in previous Halo games. So far, I've managed to work through the first two chapters (of the nine). The only really hard part so far was the part in chapter two where you have to fight through the barracks. The second dude with a gravity hammer. I found the best way to handle him is to knock him down by hand. You just have to avoid approaching him while he's in mid-swing.
Anyway, the point I'm working towards is that playing Halo in legendary difficulty is sort of ruining the experience for me. Granted, there are a few places where you have to fight your way out of a tight spot and you wind up getting lucky and doing something amazing, like surviving. But, where in normal difficulty you can run-and-gun and charge the enemy, in legendary, you're a much more timid Spartan. Ducking for cover... letting your NPC players scout ahead and take out as many as they can first. Or sniping as much as you can. It hardly feels like you're playing Master Chief. You even have to fear those little runts.
So while I find legendary mode more challenging, it's far less enjoyable experience. I had more fun with the Insane difficulty on Gears of War.