You do not have to change the bio balls ever, that is where the nitrifying bacteria live. You do have to rinse them with tank water about 2x a year but that is about it. The advantages to wet/dry is that you get a lot of surface area for the bacteria to grow on (depending on the size of the bio-chamber) The bio chamber also allows for the exchange of gasses in the bio-filter. They also allow you to add other types of filtration such as chemical, or UV. The overflow box, and filter pads on the top of the drip plate provide the mechanical filtration to catch debris.
The only cons I can come up with is that they can be a little noisy, and complicated depending on what else you add. I am putting in a wet/dry this weekend, and will have to do a little plumbing to get it the way I want it, but after the initial setup it should be easy. I am also using a steralizer in my setup wich add another pump, but I would still say it is pretty basic and strait foward.
For biological filtration there is none better. I've had mine running a year and still have not had to tumble wash my balls
yet. Just clean the foam sponges and pad. Have had mine go off for 16 hours in a power outage and ammonia was zero when i checked it 10 hours after the power came back on. (Was at work). As long as your bioload is not overstocked weekly water change of 33% should be sufficient unless you have a problem with PH being stable. Then you may need 2 water changes a week.
Pros -- efficient, quiet, easy to clean, can be stored inside stand
Cons -- Only cost, but can be made custom by yourself out of a spare tank. Regardless, imo, it is worth it.