I've done several upgrades for our clients, and really have had no issues.  It's nothing like the 7 to 8 or nine upgrade. The only things to be aware of would be:

1. There is a database update that can take some time depending on the size of your SQL database.  The longest I've seen this take is about 30 minutes on a 20 GB database.  It's an automatic process that fires off right after the upgrade, and you can't log into the repository until it's finished.

2. I have had to re-create the search index before.  Generally not a big deal since it goes in the background and doesn't stop anyone from logging in and retrieving documents.