I'm . . . . . . . . pleased with it.

'Much' is mainly used as an adjective, while 'very' is specifically used as an adverb. 'Much' and 'very' are two terms that are used often used to imply more or excess. However, the two terms differ in the manner that they are used. 'Much' is mainly used as an adjective, while 'very' is specifically used as an adverb.
As we already have an adjective 'pleased' we require an adverb which is 'very'.