As someone who didn't know about the anime or video game, I picked up Gakuen Heaven at the bookstore I work at, lured by the BLU logo and shrink-wrap. I have to say that this book did feel a little "crammed" - I could tell that there had to be more story going on somewhere.
Boy's Love seems to be a really lenient genre as far as what gets published in Japan, and even brought over to America. I have a lot of different BL books on my shelf, and not all of them are what I'd consider "keepers". Gakuen Heaven, for me, was right in the middle - on its own, it's pretty entertaining, the art is nice, but the characters are hardly developed at all - it feels like they thought fans of the game would be "bored" by the extra time that would take, so they jumped right into it.
It was hard for me to keep up with the apparent politics between the "Student Council" and "Budget Council" - I still can't figure out what the heck the problem was, who exactly was mad at who, and what it had to do with the rest of the story. When I learned it was originally a BL "dating sim", it started making a little more sense. It really feels like the plot to a dating sim. In order to fully enjoy this one-shot, though, I think you'd have to've played the series.
If you're looking just to build on your BL collection, this book should be around the middle of your list. Since it's based on a BL dating sim, it saves the "M" content for the very end. The illustrations are lovely and the book is almost worth it based on that alone, but the story only gives you a glimpse of what Keita and the school's world is probably like.