The records these guys made are fun and addictive, not to mention creative and of extremely high quality in the craftsmanship department.
The songwriters on all of their material are genius-level creatives, in my eyes. Most of their material was written by Bob Gaudio (their bass player) and Bob Crewe (their producer). Gaudio went on to have a very successful career as a producer and was a major force behond Jersey Boys as a writer, among many other projects.
And then there are those voices. There is literally nobody else in music history that sounds like this group. They invented a new niche and occupied it alone, for 60 years now.
As far as I’m concerned, they do not get nearly enough credit, even after the success of Jersey Boys and the attention that came with it.
(here is their Wikipedia entry)