I just passed Cloud+ using ProctorU. The process was terrible (delayed 30+ minutes) but my proctor said it was the first time he had seen a CompTIA exam - so thank me for being a guinea pig 

. Otherwise, I'd recommend this process to anyone since you don't have to leave your room. Likewise, it being only $99 dollars instead of $277.
I prepared by watching the free videos on SimpliLearn and I got a 7-day free trial of the McGraw-Hill book - yesterday. I would say you need to really understand virtualization as a whole - connection, storage and security tied in with best practices. I think CompTIA exams are - slowly but surely - increasing in difficulty with questions sometimes associating/incorporating 3 or 4 domains together.