Disk brake squealing after bike shop changed my pads. Is it enough reason to ask them to re-service (for free)?
This isn't necessarily unusual for fresh disc pads.

Give it a solid 20 miles. Then if you still have problems coming to a stop, take it to the shop and have them look at it. Even still, it's not necessarily something wrong with their work. It might be, if they were clumsy and didn't align things properly or left oil on the pads. But a little noise isn't enough to convict them.