5th edition. As a biochemistry undergraduate student with an interest in understanding the more comprehensive details of biochemical reactions, I found Lehninger to be very enlightening. For sure, there is a lot of information in this book. It exceeded what I needed to know to succeed in my courses. A strong background in organic chemistry is useful so that you may firmly understand the reaction pathways and molecular interactions. The mechanisms are not advanced, but having a solid understanding of electrophilicity, nucleophilicity, electron density, resonance, sterics, hydrogen bonding, van Der Waals interactions, and other fundamentals from organic chemistry will help you to grasp the material more quickly.
If you are taking biochemistry: read the book. None of my classmates read the book, and they struggled severely. Read the book. You are taking a class now where you have to read the book to do well. Find a schedule that works for you so that you can read, slowly, through the material. This isn't a story novel. You have to slow down. Read with intent. Read with intent. <-- I mean it. Take notes. Re-draw the mechanisms yourself. When you get to metabolism, clear your bedroom wall and start taping up pathways. Do it like CSI and use strings to show connections between cycles.
Overall, most of the diagrams and mechanisms are sufficient. At times, they condense the mechanisms down in a way that can be confusing. I found myself decoding "too much going on at once" at times.
* Part 1 Structure & Catalysis *
All of this material seemed very clear to me. I had no problem navigating the diagrams and concepts. Understanding entropy, enthalpy, and Gibbs energy helps a lot when understanding enzymes and protein folding. My piece of advice is to learn the amino acids and nucleic acids now. Don't put it off and slack it thinking you don't have to learn them. I found the kinetics sections to be confusing. That may just be me. I referred to an MCAT prep workbook for more basic coverage of kinetics.
* Part 2 Bioenergetics & Metabolism *
This is the hardest section. It is very dense with both "need to know" content and details for deeper understanding. I found the metabolism chapters (glycolysis, fatty acid oxidation/synthesis, citric acid cycle) to be presented well. My complaint here is that some of the mechanisms seem to try to show "too much at once" like I mention above. I am also very sad to say that I can't find sufficient information on Lactate Dehydrogenase, even though it is mentioned numerous times in the homework problems. Anyway, don't lose the bigger picture of these pathways and their regulation. You may easily recognize the effect insulin has on glycolysis, or gluconeogenesis, but don't forget about the effect it has on fatty acids. Remember to make note of those connections. The citric acid cycle: Please Can I Keep Selling Seashells For Money, Officer? Got me a top score.
* Part 3 Information Pathways *
This is the shortest but by far the most interesting section for me. If you have taken college intro biology then all this will be familiar. DNA polymerase, leading strands, lagging strands... however it gets a little bit deeper in to how all this stuff works. There are some topics that cannot answer all your questions because, well, we don't know the answer! That isn't always made clear but it is made clear enough, I suppose. My professor works in this field and his added contributions are helpful, but I think the book alone is still strong. Homing endonucleases are the creepiest things ever. Just saying.