I found the marine life to be great in Chu'uk - I actually thought my week there was the perfect balance between marine life dives and wrecks. Naturally wrecks were the constant, and the setting for every dive - but each one is covered in life.
The soft corals (dendronephthya - common name "carnation coral") are profuse and amazing. The nite dives were fantastic - and we saw literally parades of feather stars in every color imaginable. There were some really unique fish - Clown triggers, Napoleon Wrasse, bat fish, really interesting cucumbers, huge anemones(sometimes 3 feet across) hosting pink skunk clowns and tomato clowns, octopi, enormous puffers, hordes of saphhire damsels, and the rest of the "usual suspects" in terms of reef dives.
One of the biggest things that I took away from my week at Truk Lagoon, was the contrast between the man-made death machines that have literally become tombs on the ocean floor, and the vibrant life that was slowly taking over these wrecks.
Before Chu'uk, I really never considered myself a hardcore wreck diver. But my week there gave me a new appreciation for that part of diving as well as satisfied my love for marine life. We didn't see too much in the way of big pelagics... a couple of sharks, and a ray. But that's certainly no detraction from what we did see.
If you go and have the extra $100 - I really recommend doing your wreck penetration certification on the Odyssey. It was a great addition to our trip, and helped us really experience everything the wrecks had to offer. Before going, we had never penetrated a wreck, but by the end of the week we were comfortable deep inside the labrynthine engine rooms.
Before the trip, we read a lot about how the depth and technical diving requirements in the lagoon and as recreational divers, found it a little off-putting. But there was never a time where we felt we couldn't enjoy our dives due to lack of experience. There was plenty to see and do at recreational depths, and the staff of the Odyssey was great at giving us the extra training and safety precautions we needed to expand our skills.