It's similar but works different. In Spotify you have playlists you create and when you switch to "offline" mode it will download and cache the music for that playlist or whatever you plan on listening to in "offline" mode. I'm not really a huge fan of it, you can't just hit "download" like you can with Xbox Music on individual songs or albums. There is no local syncing, it's all done via the Internet. The interfaces for Spotify are great though and creating and syncing playlists actually works... and can be done via a mobile device unlike XBM right now but I'm sure MS will fix that. It has a function similar to the XBM "Smart DJ" feature but I've found I like Smart DJ a little more.
XBM has made it a little easier to listen to music in my car. With WP8 I download most of my music to my phone and when I get in the car it just starts playing via BT automatically in random mode. With Spotify on my IPhone I'd have to start up the client and in offline mode.
You can sync offline playlists on upto 3 different devices with Spotify. Spotify has tons of music but in the end it's about half the selection Xbox Music has. I have found a few obscure tunes on XBM I couldn't find at all on Spotify, but that's not really the norm.
I switched from Spotify to XBM since it's cheaper and I like the mechanics of it better. I was using Spotify on OSX which XBM has no client for right now but have found I just use my phone now even when I'm at my desk.