I swithered between three and four stars for this for a while, but ultimately, it's not joined-up enough to be a four-star read. Firstly, I wasn't crazy about Levithan's writing, which didn't help, but I can just chalk that up to personal taste. Secondly, the whole thing just wraps up far too abruptly. To borrow an Americanism, I didn't get the closure that I needed from that ending.
The characters are also various shades of "eh" as well. Will Grayson is easily my favourite, perhaps because for a long time I was the Will to my Tiny Cooper (whom I might post a copy of this book to if I can. I think it's more for him than me). However, my Tiny Cooper was a bit more well-rounded (not literally) than the character we see here. Jane is pretty much a non-entity, though I did like the Schroedinger's cat/physicist/douchebag bits rather a lot. will grayson, he of the non-capitalisation... I guess I felt that, especially at first, he was a caricature of depression rather than a well-fleshed out depiction of that affliction, and given that it's an important matter to me, I felt a little cheated by him.
It was okay. But I actually think I would have liked the book a whole lot better if John Green had written the whole thing, and I'm hardly a fan-girl of his. Here endeth the whinging.
Edit: Okay, I actually just made a bunch of assumptions about who wrote what and didn't realise that I might be wrong until I'd done it. I still think I'm right, and the general consensus seems to be that I am (at least in terms of who wrote what parts, a lot of people seem to like lower-case will, which, fair enough) but I should probably try not to make assumptions!