I did exactly the same and started the first sequel. It was guff. It's now sitting, partially read, on my kindle, and has been since May. Really annoying.
I thought the prequels were decent (but not great) and a nice read. I forced myself through the first sequel and a bit of the second but they weren't worth it.
I just finished DUNE for the first time.
I loved it and was excited about reading the sequels but pretty much everyone has told me not to bother because they're guff. I'm sad now.