So I'll preface that I'm primarily a Mac user but that doesn't mean I am automatically onboard with putting all my documents on iCloud Drive.
I have used Dropbox first, then Google Drive and iCloud Drive. I have no interest in using Microsoft OneDrive solution since Microsoft has a hard enough time keeping there Azure network up, there Live and Hotmail accounts not hacked, they have just proven time and time again how unreliable Microsoft has been in the cloud environment so far. My documents are important to me so we are going to automatically say no (for now) to OneDrive.
I liked dropbox and don't really have a whole lot of negative besides its blocked by just about any Enterprise. The plus is its not favored with Apple which has iCloud and not favored with Google Chromebooks which has Google Drive and not favored by Microsoft which has OneDrive. So Dropbox is the universal drive that works on everything and is available on just about any app that has saving to a cloud drive.
So the negative is it favors using its own Google Docs format. I learned recently that the excel spreadsheet I uploaded got converted when I used a Google Editor on my iPad. I didn't realize it was Google Editing my document that was stored on Google Drive since it didn't display anything. But as you edit your excel document it now becomes a google doc. When I tried to launch that saved file back on my mac using Excel it didn't recognize it since it was now a Google Doc. So I had to go into Google Doc and export as Excel and I had access to it again but now some of my formulas were all jacked.
The positive is if you use Gmail (like I do) for your primary email client it obviously integrates with Google Drive very well. So instead of sending the entire large attachment via email (especially since email attachments are limited to size) you can instead choose a link to the file on your google drive and give permissions. It's pretty cool and very helpful but if you don't use Gmail, then probably not a big deal.
Another pro is if you utilize Google Photos, Gmail, Google Drive, etc.. it all shares the same storage bucket and its not very pricey for all the data you can get from Google.
Obviously if you have Apple products (or an Apple ID) you automatically get 5GB of iCloud Drive for simple things like store your music, photos, Documents, Desktop, Safari Bookmarks, Saved Passwords, etc.. Which makes it nice when you log into any of your Apple devices with your Apple ID you now have access to those items. So the negative is not everyone supports iCloud Drive as a saved destination like Dropbox, then Google Drive. Also no ability to share files with anyone. The website icloud.com is slow. If you wanted to create or edit files on the website you need to your documents to be in Apple Docs format (pages, keynote, numbers, etc).
I use both iCloud Drive for syncing all my day to day stuff with all my Apple products. Its great when you get a new phone or laptop because either it got damaged, stolen or lost and you get all your important stuff back like it was never missing. I have Google Drive for big files and anything I want to share with someone else. No one else seems to do that better and integrate so well with your mail.