I quite like how the Information Commissioner's website appears broken until you find the message at the top of the screen and agree to allow their cookies... Some parts of the website work, others don't.
Essentially, you need consent unless the cookie is technically required to do what the user wants your website to do. Therefore a nice greeting (saying "Welcome Back") needs consent as its not essential, but a shopping basket (where you have forward and back buttons) you don't need explicit consent for. If you log into a website, the terms and conditions you "OKed" when you registered probably already cover consent.
The companies with the biggest problems are the site advertisers which use one cookie across multiple websites, allowing them to track which websites you visit.