Hi,
I was about to invest in a small NAS when I discovered the Sheevaplug.
I have a couple of questions I hope you guys can help with

1. Does it have an easy to use interface?
If you class yourself as a beginner, the answer is probably no. At some point you would have to use a command line interface. Unless you are used to using the command line under (say) DOS, it might take a bit of getting used to. (But it's not difficult).
2. If I buy a Sheevaplug with Debian (Squeezeserver) will it be plug and play with Squeezebox Duet?
A Duet is a standalone Squeezebox player, right? If so, the answer is yes; you can install Squeezebox server very easily:
wget http://downloads.slimdevices.com/SqueezeboxServer_v7.5.1/squeezeboxserver_7.5.1_all.deb
dpkg -i squeezeboxserver_7.5.1_all.deb
3. If i connect a 2.5 USB HDD with the Sheevaplug, will it be easy to transfer all my music to the Sheevaplug HDD?
The Sheevaplug doesn't have a hard drive as such; it has a small amount of internal storage. Better to plug in an external hhd permanently. The only issue there is that you might have to install a package so that your (e.g.) NTFS drive can be read. Again:
apt-get install ntfs-3g
I have never used Linux, so it needs to be fairly easy to configure and use.
Hope you guys go easy on me

If you are willing to learn, and the idea of learning sounds like fun, there is much to enjoy. If you aren't willing to learn, and see the configuration as a chore, well, maybe the answer is different.