We have two red-eared sliders. They're very young, about the size of a silver dollar. Right now they live in a 10-gal (I know, WAY too small) with a floating "dock" for basking, but will be getting a 100-gal upgrade in the next few months. We had some odor from the tank at first, but once it cycled and stabilized, this disappeared.
Turtles definitely have some unique requirements. It's unfortunate, but about 90% die from neglect or owner ignorance in their first year. They're very messy eaters and produce a great deal of waste, so they do necessitate regular water changes and periodic cleaning of the enclosure. Feeding in a separate container also helps. They also require a heater and basking lamp to maintain the water and air temperature. A common cause of pet turtle death is keeping their enclosure at room temp, which tends to be too low to stimulate feeding but too high to permit hibernation, and they waste away.
Turtles very commonly carry salmonella so precautions have to be taken after handling them or working in the enclosure. I've uncovered a few different takes on the story when researching them, but what I believe to be true is that the 5" requirement stems from several deaths in the 1960's that occurred after young children put the turtles in their mouths and contracted salmonellosis. Nutrition can be challenging as well, but there are a few high-quality turtle preparations on the market and the right fresh foods work equally well or better. Ours do very well on Tetra floating food sticks supplemented with betta flakes to help them maintain their color. Their nutritional needs change as they move into adulthood, so we'll have to modify their diets to reflect this.
They're very active, curious creatures. They always come over to investigate if someone approaches the enclosure. We've also noted definite personality differences between the two. All in all we've found them to be pretty interesting pets, if not something you can just stick in a tank and feed once in a while.