I registered for a Music Beta invite yesterday morning, and got the invite by the evening when I got home - so that was pretty quick, as I heard people waited a few days sometimes for an invite.
It took about 24 hours though before Music Beta authorization appeared on my Desire with the 3.0.1 Music app installed which was a bit frustrating as there didn't seem to be any way of speeding this up!
It still isn't activated on my Xoom either... :(
Other than the slow activation on actual Android handsets, I'm fairly impressed.
When driving home from work - I tested it out in the car and using the Data connection there appeared to be only one hiccup from the music and 14 songs downloaded to the cache folder.
As we seem to have rubbish signals around these parts, especially if on a call (with bluetooth headset, obviously) it can lose the signal and cut me off - I was expecting it to fail at some point.
I think my only grief with it at the moment is you need a PC/MAC to be able to upload music in the first place - I would have thought there'd be a way of uploading from an Android device in some way (even if it was an upload option from the browser...)