I love Spotify's Discover Weekly. It's incredibly outstanding. It's just about the best recommendation engine I've ever come across, save maybe Last.fm's. The mix of science and crowdsourcing behind it is fascinating; no wonder it works. It's even replaced Last.fm as my primary recommendation method, although to some degree that's because it's a push based system where I don't even have to try. It's always there for me.
Is there a downside to this? I don't really see one. In theory, people used to rely more on others to get their recommendations. This still uses your friends — and people you don't even know who share your current taste — to push recommendations to you. I actually liberally spam my friends with albums, too. I'd wager that if you were the type to do that before, you're still doing it, and if you weren't, you're getting more recommendations now.
There's a criticism that anything algorithmic is going to by its nature be "safer" and work within a set of boundaries, whereas randomly listening to college radio can break you out of your existing sonic box. I suppose that's valid, but I listen to such a wide variety of genres and popularity levels on Spotify that I don't really get that effect. Here's some stuff on the rock end of the spectrum I discovered using the playlist.
The Drink (dark twee folk-prog?)
Milk Music (YouTube commenter gets it sorta right for once: "Sounds like the Wipers joined CCR and played Husker Du covers)
Sheer Mag (whose record my friend Adam in Cleveland mastered)