Always a bit wary of these products:
1. Why is your engine so gunked up it needs this stuff, better addressing the issue?
2. If its a new rebuild and its done right then you shouldn't need it anyway.
3. If its an older motor and you use it the chances are you will remove the carbon that the motor has come to rely - the results could be worse performance/reliability.
4. What end result do you hope to get out of doing it? is it worth it?
5. How often do you intend doing it? Every 5K, 10K etc?
Maybe if you purchased a low mileage car driven by granma to bingo [where the oil never ever got warm], and it needed a fresh start in life, but otherwise I can't see the benefit.