Looker's UI is very simple and easy to use. Looker also has a lot of great documentation online which makes it simple to throw together LookML (view and model) and then build out the visualizations and plop them onto a dashboard. I've held Looker training sessions for my coworkers and most of my training materials were created from Looker's training materials; they're super easy to understand because the underlying product itself is simple to use.
I think the only bad part for me is the multiple Explores that our team has built, and not knowing which one people are still using and which ones are obsolete and therefore sometimes we have multiple views for similar things. Another dislike is probably is a very specific use case; it's hard to build out an all-comprehensive explore that can answer all KPI related questions from the company. It seems most explores are very specific to teams and therefore new users sometimes get confused where to start looking and how to go about finding the answers to their questions.
Our company is using Looker as a self-service reporting and dashboarding tool. This has already been achieved and most users at the company love to use Lookers. People that are willing to let go of using Excel have found Looker's table calculations to be super helpful.