I've been using the 50 watt Halogen back up replacement lamps for about 3 years now. Not a drastic difference over the stock bulbs, but a noticeable one. I think I paid under $20 for mine, which I think is worth it. So if you can get them at a resonable price, I would recommend them.
Another possibility that I'm definitely planning on doing in the future is to wire some small foglights, like PIAA 1500's, into the backup light circuit and mount them on the rear bumper. Rick and Matt have already done this. Of course this would be much more expensive than the 50 watt replacement bulbs, but the light output and coverage area would be greatly increased.