Are expensive HDMI cables worth it?

Yes and no.

Below around 2m length no it is unlikely to make any difference. At longer lengths yes, but not because fancy pansy "high quality" cables are better and will make the picture or sound better, but because the cable needs to be compliant with the HDMI specifications and have been tested to ensure that it performs correctly under worst case conditions.  The very fast signals that travel over HDMI cables degrade with cable length. If a cable doesn't comply with the specs there can be data loss / corruption causing the picture to be affected (bad pixels) or the cable link to simply not work. The risk with cheap longer cables is they may not be compliant at all.  If you can choose a well known brand cable who's company reputation means they will have needed to make sure the cable design is good.  If its prohibitively expensive then avoiding cables that just seem stupidly cheap is prudent. However bear in mind that the data carried over HDMI can travel at different speeds depending on the display (4K and 3D displays have much more data being sent to them compared to 720 or 1080P HD displays for instance) so if using a non max resolution display it might still be fine with a cheap and cheerful cable.  Ultimately it will usually be very obvious if your cable isn't working because nothing will be displayed or you will see bad pixels so if there is a massive cost difference there's not necessarily anything wrong with going for the cheapest and just binning them if they turn out to be no good!

Remember a cable just needs to be compliant, anything more than this, such as cables with stupid claims of being made with special metals and materials, is a complete waste of money!