The power of the preaching and the signs of Phillip would have been very impressive. I can see why Simon was attracted -- both for the right and the wrong reasons.
This is the first real preaching, action and church establishment outside Jerusalem. The Holy Spirit did not even come until the Apostles came. The experiences were so subject to misunderstanding and counterfeit that the authentication from the leadership was necessary. This Acts 1:8 expansion of the gospel was forced by persecution, but received very warmly when it came.
What did Peter see in Simon's offer to buy the ability to give the Holy Spirit? Paul accused him of being bound in the gall of bitterness. How doe those two things go together? This should be a warning to all who ask for or offer money for the gospel.