What does iCloud do well? The best thing about iCloud, as with all things Apple, is how seamlessly everything integrates. It's awesome to know all my tech will work together and talk to each other. And I pay for the extra iCloud storage for peace of mind, and never running out of space for photos.
What does iCloud not do well? I have a very old iCloud account, so I have the email address to go with it. This is sort of a hassle since I never use it, and I've had to reset the password a couple of times to retrieve info that I needed. It's an extra step I shouldn't have to take, but Apple loves Apple.
What do you wish you had known about iCloud before using? I wish I'd known about the changes to customer service over the last year or so. They've made it so much easier with options to share your screen from home, etc. It was a pain to have to try and talk to customer service or go into the Apple store to get iCloud and device issues resolved. They've really streamlined everything, and updated their process, so it's very easy to get through to them and get the help needed. I've been very pleased with iCloud.
Have you used a competitor to iCloud? If so, how does it compare? Not really. I only really use Apple devices.