I've got the classic case of toomanybookitis.
I've been using Rapid Review Biochem, Kaplan Biochem (I'm taking the class), Lippincott's Biochem and even HY Biochem on and off.
My thoughts:
HY Biochem is worthless (I think I had the previous edition to whats available now, but I doubt the newest one is any better... the book is completely inadequate.)
If you're in the course, Kaplan Biochem is excellent and has nice large diagrams that help put everything together. It focuses on the highest yield diseases (e.g. G6PD deficiency), but tends to gloss over many of the others (e.g. maple syrup urine disease). If you're not actually taking Kaplan, then I think the book is a bit inadequate. You're going to need another reference to supplement it.
Rapid Review Biochem (2nd edition version) is good, but feels a little heavy with extraneous information. You can see Goljan's fingerprints all over it as they incorporate virtually everything into a clinical presentation. But sometimes the chapters get a little unorganized. And it doesn't do a very good job emphasizing key high yield points. I'd recommend this only if you have plenty of time to review.
Lippincott's. HUGE, but extremely thorough. I actually really like this book, but again... only if you have plenty of time in advance to review. At this point, I only use it as a reference in instances where my Kaplan book doesn't do a solid job explaining things.
I think its important to avoid choosing a reference based upon the types of questions they may or may not have in it. You have qbank and exam masters for that sort of thing.