Getting a user interface correct is very difficult because you have no idea how people will use your user interface until they do, and you have no idea of the habits they will get into while using your interface. Plus, getting to grips with bugs is a big problem.
A well-functioning user interface goes a long way towards the best possible user experience. You need a company that can harmonize between function and experience, while hopefully creating something that is attractive.
It is a tough job, so we were especially careful when testing out UX design companies. We at PeakFirms have ranked and listed the best we could find.