As much as I like iCloud, I still need to use a file storage service such as Dropbox because iCloud's offering is so limited in this area. It will only store and sync files created with Apple's iWork suite. I'm a big fan of Dropbox, and it's likely the most popular file-based cloud service. This is no doubt because it's utterly simple to set up and use. But it has some serious competition in Microsoft's SkyDrive and the new Google Drive. In every case the main advantage of these services is that they automatically sync your files among all of your devices. Any file I put in my Dropbox folder on my computer is automatically updated to the cloud as I make changes and is automatically available to me on my iPad.
But Dropbox has a serious disadvantage: you can't use it to edit documents. While that's not the primary purpose of a file-storage/sync service, it can be a real advantage. If I'm working on an article for iPhone Life magazine, it's handy to have that article available to me for editing on any computer connected to the Internet or via my iPad. Both SkyDrive and Google Drive offer this capability.
However, while both Dropbox and SkyDrive offer iOS apps, Google Drive isn't yet available for iOS. Google does say that it's coming soon.
iMore has posted a great article comparing SkyDrive, Dropbox, and Google Drive for iOS users. They compare them in terms of interface & experience, features, workflow and syncing, third party app compatibility, and storage space & pricing. It's a very helpful article if you're wanting to use a file-storage-and-syncing service on your iPhone or iPad.
Overall, they like SkyDrive, but they're very clear in saying it depends on which features you value the most. And that's why this article is so valuable. It helps you decide which features matter most, and then make a decision on that basis.