They expected to be better off the moment they invested in but the idea of bitcoins is not as they think, hence they will tag it as a ponzi scheme or fraud.
Bitcoin is definitely not a ponzi scheme because nothing about Bitcoin is a secret. It is one of the most open technologies in the world. Its open source, anyone can review the code, anyone can contribute to the code, anyone can run the software voluntarily and participate in the network, and anyone can use the network without permission. The entire history of all Bitcoin transactions is visible to anyone in the world too. So where is the ponzi in such a very public bitcoin system?