Creatine aids in the availability of ATP (adenosine triphosphate) to the muscles. You get energy from the conversion of ATP to ADP (adenosine diphosphate). The body puts back the extra phosphate at its leisure to turn the ADP back to ATP for later use.
Subjectively, after about a week you suddenly notice that you have tons more energy in the gym and you will finish sets long after you'd normally have given up with fatigue. You'll also get a little bloated which will add to your strength, enabling you to lift heavier. Lifting heavier will give you keepable gains.
When you come off the creatine the slight bloat and the energy boost will go and you'll lose some of the strength of being on from the lost bloat but if you used the extra strength and endurance to good effect then you'll come out of the other side better than you went in. There's no pct required but the gains don't compare in any real way with gear. Think of it as a boost to your workouts which, if you make an effort to do more with it, will make you stronger. I usually run a five-day loading period followed by 4-6 weeks at 5g per day and then take 2-4 weeks off. Some people suggest creatine as a way of just pushing yourself through a plateau.
So, in answer to your question, yes, creatine will give you gains within 2-3 weeks. Coming off will not be a problem. How much better you are when you stop depends on how much extra work you put in while you were using it. If you want real long-term gains then you need to find a decent long-term workout program.
Remember to drink lots of water through the day when taking creatine.