With lots of gambling sites and sportsbooks I'm currently using, I would say that almost most of them have a good user interface. I just wondered if there are any most used gambling sites right now that have a not user-friendly UI. I think most of the legit sites today do have a good UI and are organized well. If the site has a trash UI then don't expect them to have a good service as if they can put an effort to make a user-friendly UI then what's more to the service?

Improvement? As long as the interface is not messed up, that's good. Rather than UI improvement, it's better to hope instead for much better services like keeping good customer support, maintaining the security of the site on the top level ensuring the safety of user funds, and always bringing up catchy bonuses and promotions. These things are a big factor to consider in luring more users to a site.