A good overview of considerations here. In particular, this bit particularly resonated with me:
I also secretly judge [agencies/freelancers] based on whether or not I would actually hire them for work. Many designers with a strong sense of aesthetics are lacking in the UX department, and their site is nearly impossible to navigate. Sometimes the the UI is easy to navigate, but there are possible functional problems. Sometimes they’re issues that could easily be solved with progressive enhancement, but no one bothered.
There are two reasons for this: bandwagon-hopping, and misplaced experimentation.