You'll still get 250 non-Amazon songs for free. But mostly it looks to be a competitor to iTunes Match now.

I just logged into Cloud Player, and it gave me 30 days of Premium for free. Then it ran through a process to add/upgrade songs.
I believe those "581 past Amazon MP3 purchases" were ones I purchased before Amazon Cloud launched (previously it only added new purchases automatically).

Edit: My Purchase History in Cloud Player now goes back to June 2008.

I just bit the bullet and signed up for the $25/year plan. I knew the unlimited-storage thing wouldn't last forever, but I'd uploaded 20K songs, and have just started using it at work, where it's much easier than trying to store a ton of music on a physical device. I guess I'll see if it continues to be worth it.

This is why I opted to get a Droid 4 instead of an iPhone. I LOVE being able to store me entire itunes library for free with the Google Music cloud and play them back on my phone or internet browser at work.

I use this on my Droid as well. It's WELL worth it.