I use two difference services, Amazon and Google. I uploaded all 300 GB of my music to Amazon and I can stream it at any time. With Google Music, I've uploaded only a select part of my library.
With Amazon, there's a definite lag that comes with having such a huge library on the cloud, since it takes a bit longer to browse through the long index of artist or albums or genres. The screen would go black, trying to load the previous page or the next list of albums, etc. But, once the streaming begins, it's seamless most of the time.
With Google, streaming is quicker, and so is the browsing experience, though it is a considerably smaller library.
The uploading experience varies greatly between the two services, but I won't talk about it right now.