I'm doing the 30 day trial right now, and based on my experience so far I think I'll let my Sirius plan lapse in April and switch to Slacker. The SQ is nicer, having more control over the music is excellent, and the price is better (not to mention that Sirius Canada is going up $1/month in March due to "Canadian Music Royalty fee".) The only thing I would miss from Sirius is Radio Classics, however we have a local AM channel that plays old radio shows at night so I could always setup a recording for that.
Right now I'm using the app on my Blackberry Bold 9700 and it works pretty good, however I keep experiencing "lagging" in the songs (slows down for a second then speeds up.) This happens only while driving, but not sitting in the office or at home, and even while playing cached stations; so I think it's something to do with switching cell towers (e.g. causing CPU to slow down.) Now that I'm typing this, I realize I was playing it on my car stereo through Bluetooth, so that might have something to do with it; I will have to try hardwired on the way home tonight.
Also sometimes the BB App seems to mess up and start playing two songs on top of each other, or won't play at all until I shut it down and open again.
The only other thing bugging me is the lack of normalization; some songs are really quiet.
However I do love the automatic caching over wi-fi (gets triggered when I plug it in to the charger.) And it would be nice to just have one device; many time I forget my SL2 at home.
So far Slacker seems like a winner.