I use side reins to lunge my horse. I personally think they're a great aid. My mare doesn't like to move straight, she like to twist a squirrel her way out of moving correctly... mostly because she's lazy and it's easier to squirrel around in circle, then to frame up and move out. I love side reins, because the help the horse move in the correct direction/position, and encourage the horse to move into the bit. I love the elasticity, it's really helped my horse soften.
As Sara said, I wouldn't start you horse with them just yet. Get him to where lunging is embedded into him before you try anything with him. And when you do incorporate side reins into lunging, start out very loose, so that in the beginning, they're just attached to him, not really reinforcing anything, and slowly, you can bring it up, one hole at a time. Don't try to get him into any sort of 'up' frame... you'll run into trouble. You want the side reins to be soft in his mouth, and to encourage rather than force him to set his head and work through his back.