What does iCloud do well? iCloud is Apple's in house cloud storage solution. It does a great job of linking up all of your various apple devices and giving them a shared storage cloud. You can also access your files through any browser. Works wonders as long as you stay inside the Apple family of devices and operating systems.
What does iCloud not do well? iCloud makes buying and selling used or refurbished Apple products very complicated. Be very careful to fully disassociate your iCloud account with old devices. iCloud is not as widely integrated into other programs and operating systems outside of the Apple family.
What do you wish you had known about iCloud before using? That iCloud has reoccurring payment options for more space that will charge directly to your iTunes account if you would like. You can add space for as little as a dollar a month!
Have you used a competitor to iCloud? If so, how does it compare? Dropbox. Dropbox works on more devices and is not as limiting as iCloud. If you dabble outside of Apple hardware Dropbox might be the better option for you as it is more widely supported.