This is my second modern bike and I got the same problem each time. The chain slips when under load (usually when I'm standing, but I can slip starting at a light and still sitting too). At first I had an aluminum cassette and the dents broke quickly. I replaced that old cassette (that actually came off as it completely broke) with a steel cassette.
The steel cassette has been working well in the sense that the dents are still all looking good and it has not come into pieces yet, however, the chain still slips.
In both cases, I had 9-gear cassettes when I use the smallest gear 99% of the time (San Francisco type of hills still require me to change gear, otherwise, I rarely change anything, so I could assume that the small gear of the cassette gets worn down quickly...)
Would it be possible to have a cassette with longer dents to avoid the slipping? My old steel frame bikes had cassettes with really long dents, in comparison. Could a find similar cassettes that are compatible with my new bike? (I have a specialized Roubaix now)