SkyDrive uses your live.com credentials. You can link multiple live.com accounts together. The desktop integration is improving. Recent versions have added expected features like being able to hide the dock icon. I have had to stop using it because of problems with the Keychain and getting it to reliably sync.
Google Drive uses your Google account or Google Apps account as logged in via Safari. This can be quite annoying as I have multiple accounts and I do not use Safari as my primary browser. Google Drive can get upset if it looses your account information and refuse to sync with it directory forcing you to have to download the directory again from the cloud. I’m still using Google Drive for limited items in a wait and see model. It has otherwise been stable.
DropBox is not tied to any other credentials system which for me is actually an advantage. They have recently doubled the available storage for dollar. I pay $99/year for a ridiculous 125 GB of storage. I love their new link feature allowing me to share a file with a person who does not have a DropBox account easily.
With any of these services you have to remember that you are using a third party to store your data. I use locally encrypted sparse disks to store anything that might be sensitive. I run a program called Identity Finder to scan my hard drive for anything that may contain sensitive data like old tax returns, health form or credit card receipts.
