I have a Power Now on my KX500 (same part as for KTM 125SX, since it also uses a Keihin PWK 39mm carb). I think it is worthwhile; it doesn't change peak power at all, but the bike is more crisp at partial throttle openings, and feels "snappier". And that was with proper jetting before installing the Power Now, plug readings showing that jetting was ballpark.