Well ideally you'd give them a price too, I suppose
But yes, it is essentially as simple as that. If you want to get more technical and manage your own money, then you would install the client and deal with your wallet.dat file yourself - Mybitcoin does that for you, in exchange for you trusting them. But that's more advanced stuff and if you want to just get started right away, then use Mybitcoin
I've got nothing bad to say about Mybitcoin, just to be clear. They provide an excellent service and without them I'd imagine it would be much harder for many people to get started.