As you would imagine, the big dial allows for you to switch between the various filters that come with the case. This would give you 9 different effects to meddle with, although it does not appear that you can purchase additional filters. However you could combine this various lomo-esque apps such as Instagram and Hipstamatic to give you even more trippy and psychedelic results.
The drawback to the Holga iPhone Lens Filter Kit is that it appears to block off the flash on your iPhone 4/4S, and it doesn’t look like it will fit nicely into your pocket, meaning that you will probably need to carry this case around with you separately. However if this has piqued your interest, the case is available from HolgaDirect for $25 and will be compatible with both the iPhone 4 and iPhone 4S.