80% of the tracks on my account are private. I mostly use Soundcloud as a musical post-it system so I can easily listen to my productions in different settings (car, friend's hifi system, earphones etc.), share tracks with label in a "secure" fashion.
I was really saddened to see they introduced ads even for people who actually pay for their service, I felt literally betrayed and angry but fortunately it did not last long. However it leaves traces.
I still find it amazing that they did not introduce basic filters for the stream, like "just grab house mixes at least 45 minutes long"...
Anyway, I am still quite happy with the service even if I could do the same on my own dedicated server, minus the social network thing.
I just wish them good luck but I hope that their longevity will not be due to ads served even to subscribers...