Got it because of said deal (had free 30 day PSPlus subscription, so figured, hey, why not?). The quantity of music seems pretty much on par, and being Sony, they have a huge classics selection. I also tend to find more smaller artists on Music Unlimited than on Spotify. I'm not big on mainstream artists, but they've had everything I've looked for so far. And their artist-based radio delivers a much wider selection than Spotify, but isn't as good as Pandora. Related music/artist discovery is so-so. Spotify tended to be better on that regard, as well as with supplimental metadata like lyrics, bios, etc.
What will really get you is on how you access the service. Their web-based app is flash, but works well. Not much in the way of social integration if that's your kick, but it's plenty functional for me. No bugs, hiccups, or anything. Their iPhone app is...well...crap. It's got a slick interface, but is buggy as all hell. Sometimes it wont' connect, sometimes it randomly stops streaming never to resume again without a phone reboot, and if it looses connection to your headphones/bluetooth device it'll just resume playing on the speaker rather than pause. I've even had it stop playing when it lost focus until I killed and relaunched the app. And it doesn't have offline track support.
By contrast, the Vita app works great, has offline track support, and is snappy at streaming and interface. Ironically, we have a Sony bluray player that my girlfriend was gifted and a PS3 which both support the service, though I haven't tried the respective apps.
One nifty bit is that they have a helper program for your computer. Point it to your MP3's, it'll scan them, then automatically add them to your online "Library" as matched songs ala iTunes Match or Google Play. No downloading, though.
At the moment I use it as a replacement for Spotify, and all I really used Spotify for was streaming specific artists or tracks that I didn't have in my library. And for the occassional music discovery. For $12 a year it's worth dealing with the buggy iPhone app, and it's radio/artist/genre streams have been good enough I haven't subbed to Pandora yet.
They have a free 60 day trial in the US PSN store (dig around, there's a 30 day too that's easier to find). Not sure about the Canadian PSN store, but I don't see why they wouldn't.