Have just uploaded 2.2k songs.
The first 1.5k went pretty fast, so I guess the scan and match is working and it uploads what it can match first, then manually uploads the rest.
I made sure my collection was complete and had full metadata and album art before uploading, to avoid having to organise on google music and it seems to have worked pretty well. No errors that I've found so far.
I'm pretty impressed by the service, and have been wanting to move my music to the cloud for a while now. This has given me the excuse to organise my media, back it up and free up vital storage space, while giving me the added bonus of synced collections across all devices.
Now if only google would start a spotify-like service where I can stream anything I don't have in my library for a subscription fee..