If you have the money to buy the 5 gallon, I would HIGHLY recommend doing so.
I have a bit hesitate to buy some the first time because I was thinking it would be more like a quick detailer. Boy I was wrong.
I use it primarily as a waterless car wash. Ive cleaned some pretty dirty spots on my car without any marring and im blown away.
If i had the money to order 5 gallons I would not think twice about it. You can make back the money you use S&W with by the water you dont use washing your car is how I look at it, not to mention is takes 1/10 of the time to clean your car.
If I had to vote for the best product of the year, it would be hands down S&W for the versatility of the product.