I've been uploading for a couple of weeks and was nearly done until I hit a major bug: the music manager says that I've gone over the 20K mark but I've only uploaded just over 10K. I submitted a report and, amazingly, they responded in less than 24 hours: "We're aware of an issue with larger libraries that causes users to prematurely hit the 20,000 song limit, and are working on a fix."
As suspected, the 20K song limit was an estimate. In reality folks were originally given an allotment of space that was supposed to be the equivalent of 20K songs. This caused a bunch of confusion as some folks like me were allowed over 20K songs, and some folks with super high bit rate/large files were limited to less than 20K songs.
As this played out, I was originally given the right to upload about 23K songs. As I went to clean up my collection, I was then not allowed to upload anything more until I reduced my number of songs below 20K, so it looks like El Goog is going to be flexible on space, and go by the song limit. At least for right now.
I've actually got a lot more than 10K, maybe over 20K, but I'm only uploading 320kbps AACs newly created from ALACs that themselves were created for my ongoing re-rip project. I convert a few hundred tracks at a time and direct them to a dedicated Google folder. Once uploaded, they're deleted and process is repeated. I put the track titles in various playlists to keep track of progress. It was going swimmingly, until....
This is a similar process to what I did. I basically deleted my entire upload of 23K songs and am now going through my collection album by album to upload. It's rather therapeutic and allows me to clean up a lot bad data.
My point here is a caution: use this move to the cloud to clean up a lot of your data. I spent two days cleaning up my contacts data, photo's and now I'm cleaning up my music. If you've got a mess in your multiple hard drives, cleaning it up before you move it to the cloud will save you so much headache.
As for actually playing the uploaded tracks, well, the web player won't work on my work PC (an "oops" error pops up), whether in IE or Firefox. I tried installing Chrome but some unknown error causes it to fail. We're upgrading to Win7 next month so I'll just wait and see until then.
They will play on my iPhone through Safari but the web player is very mobile unfriendly. Maybe Google or a third party will make a mobile version--not an app but an HTML 5 website for mobiles. I definitely notice a dent in sound quality, however. I don't know if Google actually streams the AACs I uploaded or if they're transcoded into something else.
Those all sound like local errors as I have tested the web version on IE7-9/Firefox/Chrome across WInXP/Vista/7 and various linux derivatives. They've all played fairly well, with the occasional glitch when I'm doing high bandwidth tasks.
As for playback, it seems to be capped at 320 Kbps so you could be in the sweet spot. I wish I had done the same when I started ripping.