Q:

Are there any downsides in using 2x extenders?

What are the negatives with using teleconverters?

A:

In general this is what you can expect for using a 2x extender:

Lower max aperture — which may cause your camera's AF system to stop working, or at least perform worse.
Darker view finder
Loss of sharpness, especially at the corners
Loss of color contrast
Increase in color fringing and chromatic aberrations.

The effects of the optical degradation will depend on the lenses you pair the 2x with. Also, some Canon lenses aren't directly compatible with the 2x TC, though there are "mods" that will allow it to work with many lenses.

A:

There is also a huge difference between an adapter that screws onto the threads at the end or your lens and a professional grade teleconverter that fits between your body and lens.  You still lose a lot of light, -pro teleconverters will usually tell you how many stops specifically- but the optical quality of pro TCs can still make for minimally distorted shots.  
The sad truth is that you usually get what you pay for with these things.  I'm not saying you have to buy name brand, but the very low-end versions of these are essentially worthless if you have any intention of trying to sell or publish your photos.

