Is ClearBoost just another "sticker" antenna?

No. ClearBoost is a real antenna, and actually works. The antenna is designed in two parts. The copper sheet visible inside and at the bottom of the case (which only superficially resembles the infamous foil stickers) is sized, shaped and positioned for maximum capacitive coupling with iPhone's internal antenna. The rest of ClearBoost's antenna is embedded within the plastic case, up through the back, into the "stub" at the top, and is "tuned" to work best within the 824-894 MHz band.