A 2x teleconverter gives more reach, but the trade-offs are significant.

Main drawbacks:

• lower maximum aperture: a 2x converter costs a lot of light, which can make autofocus slower, less reliable, or stop working on some camera/lens combinations
• darker viewfinder: because less light reaches the camera
• reduced image quality: typically less sharpness, especially toward the corners
• lower contrast and color quality
• more chromatic aberration/color fringing

How bad it gets depends heavily on the lens and converter. Good rear-mount teleconverters made for compatible lenses usually perform much better than cheap front screw-on “tele” adapters, which often have much poorer optical quality.

Compatibility also matters: some lenses simply don’t accept certain teleconverters without modification.

In short, a quality teleconverter can be useful, but with a 2x model you should expect noticeable light loss and some optical degradation. The better the original lens and converter, the better the results.