Update: I now use Google Drive, Dropbox, and Amazon Glacier to back up my files, each for a different purpose. I still love Dropbox as my favorite syncing/sharing service, but I use google drive with my work account for sharing files in our network, and Amazon Glacier as an emergency only Fire-safe backup for photos and other stuff.
When Google Drive came out I was initially really stoked. I love google, their image searching features seemed awesome, and their pricing is super competitive. However, after using it for a number of months, I had to finally switch back to dropbox.
Ok, if this isn't the biggest reason to switch file backup and syncing services: I don't know what is. Shortly after installing google drive there was an error. I had to re-install google drive, and then when i choose the same path it started doubling all my files. I wasn't sure which ones i could keep or which ones I could delete, because the folders weren't identical on my multiple computers anymore. At some point I reached my maximum space limit (because all my files were being doubled) and google drive stopped syncing.
In addition to this, now that I'm getting off I'm finding that deleting folders off my computer is also permanently deleting google docs that I wanted to save, even though Drive is not supposed to be syncing google docs files. These type of errors (in addition to ones mentioned below) have really shaken my confidence in Google Drive's reliability. Do I want shaky confidence in my backup option? No.
Google Drive can typically use 4 to 10 times the amount of memory that dropbox uses, even when it's idol. This is super annoying, especially when i'm running a memory intensive bit of code or doing some image processing.
With Dropbox, I never even realize I'm using their service. It works seamlessly Google drive is constantly telling me to re-sign in. If i forget, or ignore the message, then my files aren't syncing.
When you delete a file on your computer using Google Drive, it's still taking up space in your drive until you visit the web interface and empty the trash. And for whatever reason: Emptying the trash doesn't always remove the files. Buggy.
I did an informal speed test, uploading a huge folder (~8 GB) of pictures from my laptop to my desktop using Google drive (which got a head start) and to Dropbox at the same time, over the same wifi network. Dropbox crushed google drive
In addition to these factors, dropbox gives you lots of ways to get free space. When you upgrade your account, they double this free space, which adds a significant chunk to my storage capabilities.