Softlayer's cloud is reliable, but the I/O performance of their cloud SAN is really low. Even standard single I/O operations (running rsync, copying a large file, performing a database dump) tend to stall the server in a massive IO wait.
As for the best cloud host? I don't think there is a "best" cloud host. Each cloud has its own unique blend of characteristics. A cloud that might work well for one application might prove insufficient for another. It's worth doing the following:
- Look at the list of features each cloud offers and determine if it meets the need.
- Get a test account or a temporary instance and benchmark your software on each cloud.
Speculation is good, but nothing beats a real test.