I destroyed my credit when I was younger. Since I have had this card, I have been able to attain credit from more reputable companies. This card has improved my credit score, as promised. That being said - Credit One is expensive and they will nickel and dime you until your head spins. Relying on this card will probably ruin your life so be very careful. If you use it, go home and pay it off immediately. Expect random fees constantly - and pay them. Don't even bother complaining - your complaints will be heard by a guy who probably lives in a mud hut in India (they out source customer service). They simply will not be sympathetic to your first world problems.
This line of credit has helped me get approved for better lines of credit, thus creating a chain reaction of credit healing. I use the analogy "playing with fire" because fire is very usefull, but the second you are not careful it will probably burn everything you have down to the ground. So be very careful and understand who you are dealing with.