I went with 2 - 500gb Seagate Barracudas. As for how it acts, I have them mapped on every computer that is running on my home network.....basically they act as just another drive. Every PC on my network uses it to not only back-up but to also access MP3 files and video files.
I also use it with my SD Transporter to stream audio without any problems.
As for FTP, I use it to let other family members (spread out all over the world) access our home movies, pictures etc. I also use it when I'm not at home and need to access something.
Once you get the hang of it, it's a great system to have.