Riding on the beach below the high tide mark is like riding on a really smooth road, once into the tracks its almost impossible to say what it will be like - rain, wind and the amount of other traffic will dictate how churned up the tracks are. Watch out for creek crossings on the beach as they can be deep even at low tide. The biggest hassle will be other 4x4s who wont expect to see a bike and drive like lunatics.
I was at Inskip Pt last week and the tracks into the camping areas and beach ramps were all deeply rutted, the beach above the high tide mark was very soft and churned up. It was very busy, 2 - 3 hour queues for the barges, 20 to 30 minutes to get off the beach - busier than Melbourne!!
It will be busy in Jan during the school holidays - best book your camping now, if there's any left.
I've been over a few times in school holidays and always found the camping to be crowded and noisy. We've also done a few day trips from Rainbow Beach and Noosa onto Fraser using 4x4 buses and a rented Landcruiser. I quite enjoyed the buses, got to see a lot more from high up, the driver knew the best places. Downside is obviously no freedom to wander off and you can only visit the Southern quarter of the Island. If you can find other backpackers at Rainbow Beach then a rented 4x4 can be a good option.
You can have a trial run in Cooloola National Park before getting to Inskip Point (before forking out for vehicle permit and camping permit) - the tracks will be the same as on Fraser. If you cant hack Cooloola then you wont cope with Fraser.