I think whether or not you enjoy Just One Day / Just One Year hinges on how magical you thought Lulu/Allyson and Willem's "epic one day in Paris" was. Half of Just One Day and all of it's companion, Just One Year, is the aftermath -- how much their lives and the people the become changes after spending that one day together.
I thought the concept was really intriguing, but unfortunately I just didn't find their day together that memorable. Both books were just kind of meh. And it didn't help that I could not relate to Allyson at all. Man, if I were 18 and touring Europe with a group of people my age, I definitely wouldn't be dragging myself from place to place, spending every night hanging back at the hotel with our middle-aged guide. If I had a one night stand with some Dutch hottie in Paris, I'd be congratulating myself on my spontaneity, not spiraling down into a state of depression. Allyson literally cannot deal and I'm just not the kind of person that sympathizes with her behavior. (You know who else falls into a drama coma when the most perfect of perfect men disappears? *cough* Bella Swan *cough*)
So, needless to say, it was reeeeally tough for me to make it through her book. I get that it's a story about person growth and coming of age and all that, but maybe those types of books just aren't for me. Apparently I just don't have the patience -- I like to read about heroines who are basically awesome and kick-ass from the start. And that's okay too.
Willem's journey in Just One Year was slightly less painful... but I didn't didn't really care about how his story turned out. I really don't think that Allyson and Willem are characters that will stay with me. This will probably be a series that, eventually, I'll forgot that I even read.