If you want to use the standard theme it's very easy to implement. It takes no more than an hour. The problems start to appear when your client wants something a little bit more special. Things that may seem very simple to do will give you the biggest headaches. Trust me, you will curse the Magento developers a lot at the beginning. You will probably get paranoid and think that they made it so hard to customize just to make more money by charging for support (200 per hour or something like this). And maybe you wouldn't be so paranoid after all...