I have used them in my games, and the characters have said "Wow, these guys feel more effective than we are", and the Sidekicks were of a lower level. I think it had to do with the fact that the Sidekicks are actually focused on what they do, and do it well.
I have had several players ask about playing Sidekicks as primary classes, and its something I've mulled over. I would use Sidekicks alone for classes, I wouldn't mix them with standard classes. The Sidekicks would then allow us to organically add ribbons or other small things to customize the character, compared to everything that gets dumped on you with the 5e classes and subclasses.
I've also had a couple players ask about "simpler" classes for them to play. They really don't feel like learning all the stuff that can go into a 5e character. They just want to wander around and adventure.