RedBike said:
6 months is more than enough time to wear out a chain / cassette. It all depends on the sort of mileage you've been going.
Well yes it does, strictly speaking. But you'd have to be really caning it to wear out a chain/cassette in six months. I do 75 miles a week year round on London's filthy, gritty roads, and my chains/cassettes last a helluva lot longer than that.
I'm with earlier posters - unless they gave you the service for free, I'd definitely go back and say 'it's not working right'. If they're any kind of a decent shop they'll want to put it right - and if they did charge you, they pretty much have to.
At some point you might like to go and have a look around cycletutor.com, and have a think about DIY. It's actually a lot easier than you might think, and very satisfying. You can save shitloads of money too.