The main advantage of a wireless speaker system is pretty obvious -- there are no wires connecting the speakers to the sound system. You can put your speakers anywhere within transmission range and not have to worry about tripping over or hiding wires that lead back to your sound system. This can simplify setting up a home theater system.
Wireless speakers are popular in outdoor speaker systems. If you want to set up a speaker system on a deck, patio or swimming pool area, a wireless system may be ideal.
There are a few disadvantages that come with wireless systems. Because wireless speakers still require power, it's likely that you'll need to plug each speaker into a power source. Battery-operated wireless speakers exist, but may not have the audio oomph you want from your sound system. If you need to plug each speaker into a power source, you may still find yourself limited in how you can set up your home theater.
Interference can be another problem. There are lots of devices that emit radio waves. If those radio waves are on the same frequency as your sound system and speakers, you could get some garbled signals as you listen to your music. Dropped signals can also be a issue -- if a transmitter or receiver stops working or drops out, it will affect your listening experience.
Another problem is bandwidth. Wired speakers can carry a lot of information in the form of electrical signals. Wireless signals can't really compete. Music may seem less full or rich. It's a subjective element that can be difficult to put into words. If you're an audiophile, you may find wireless speakers lacking from a performance standpoint.