I posted this over on Slacker's forum, just thought some of you maybe interested.
I signed up for a Premium account today. Heck I've tried all the others so why not.
My early observations are as follows:
Library wise they have some gaps, but I suspect that will be fixed pretty quickly since they are just launching.
iOS App is smooth as a fat babies behind. No lagging, very easy to create and edit playlist on the fly. Don't have to be playing a playlist to edit it. The one thing though that Slacker has is that you can see if you've already added a song to the library, but not so with Spotify. The software though is well designed and very well thought-out for ease of use.
The Windows and Mac Software works great without having to use the web browser and is honestly amazing in comparison to Slacker web based interface stuck in that small window in the middle of your browsers.
I can use Spotify on all my iOS devices, Mac/Win Software and download my playlist to every single one of them. You aren't limited to one device. You can't stream but on one device, but you can download/cache your playlist. There is no native iPad App so advantage Slacker there. But Spotify has the advantage with being able to download in-demand music to all my iOS devices and my desktops, with full Mac support.
Radio stations, again Slacker has the advantage and it will be the reason I will continue my plus subscription.
Not available in Canada vs Slacker which is. Doesn't impact me!
Slacker's $3.99 per month with radio station caching (unlimited devices) and customization in my opinion still gives them a big advantage over Spotify. Really they have no answer for this.
Overall I am really liking the service, but time will tell. My biggest complaint with Slacker right now is the lagging slow application, which you only notice if you have the Premium subscription. If you use the free or plus service you don't see the lagging between playlist, stations and albums.
I think Slacker is in pretty good shape to compete here. The one thing Spotify is going to do is market, market, market. Slacker doesn't market at all that I see. The release a few PR's here and there, but that is it and that is a big mistake.