My great white whale from the late '80's through the mid '90's was the Leonard Wolf annotated Dracula. In the process of finding it I saw the corresponding volume for Frankenstein and picked it up. While I think I've read the book before, I decided to read the annotated version last fall when Collin & Parker were reading it for their sophomore English class.
The annotated book is overkill, but it is how I like it – I just love annotated books. Every odd fact gets an annotation that explains the historical setting and what Shelley was referring to. The one objection I had was that what initially attracted me to the Dracula book was the haunting woodcut-like artwork of Satty. The artist here is Marcia Huyette, and while it is good work, it has a '70s vibe that I found distracting.
But overall a great edition of a great book – even if it's since been superseded by more recent annotated editions, including by Wolf and Leslie Klinger. But do start with the 1818 Frankenstein – Shelley's later edition is some pretty significant revisionist telling of the take.