The only real drawback is that you won't be able to use them when you buy a Canon.
Seriously (folks), extension tubes are a really good idea and the Kenko ones are among the best. Tubes don't degrade the lens performance because there's no glass in them.
Yes, you will be able to focus really close and get 1:1 magnification. However, you won't be able to focus at any distance - the range will be very restricted.
You can use any number of tubes in combination to achieve whatever focus distance you want.
Auto exposure and autofocus are preserved, BUT in general manual focus is the way to go, and often it's easier to set the focus ring and then move the camera backwards and forward to get what you want in sharp focus. Some people use a macro focusing rail on the tripod so you can move the camera precisely. Bear in mind that the DoF will be tiny unless you use small apertures, in which is will just be small.
Go for it!
