Just got back from our 4th stay at Pop Century last week and absolutely LOVED it again!!! We got to stay in the 50's section again (my favourite now) on the fourth floor facing the pool and I would highly recommend this section. Not only is the bowling pin pool very cool and not very crowded, the whole section just seems to be a bit quieter (as ThinkTink721 said).
Also, the food court has an awesome menu that kept my family of four (two teenagers) satisfied every night we ate there (probably 7 nights out of our 10 day trip). And we didn't find it too busy most nights, although we were coming back from the parks and eating between 4pm and 6pm most nights before heading back to the parks (and this was during Canadian Spring Break...very busy down there)! The Everything Pop store is also very cool and always well stocked with a great selection of merchandise and goodies. And like everyone else has said, the buses are probably THE best onsite, even though the stops for the ride back ARE way out at the end usually in most of the parks.
I honestly can't think of anything about Pop my family doesn't like and would stay there a fifth time in a heart beat!!! :shrug:
EDIT: Not sure how important this fact is, but if you want two rooms that are side by side with the door in between, I think you have to request connecting rooms rather than adjoining. I know the first time we requested adjoining rooms, we were given rooms on the same hall, but down the hall from each other. We were told at the front desk to ask for connecting rooms the next time, which we subsequently did each trip afterwards.