The topic has been discussed a number of times on some of 4WD / offloading forums I have been a part of. While some people swear by it, I'd say the general consensus was to to avoid it and a quality paint would offer plenty of protection. The biggest issue cited was that, given time, the undercoating would begin to crack and would (or at least could) trap water, salt, and debris between the undercoating and paint which would actually accelerate corrosion.
Perhaps some of the newer coatings are much better and are worth it, but I wouldn't spend the money.