In general, as an ex estate agent, I would recommend allowing the agent to do the viewings. However you do need a GOOD agent, and from what I saw when I last moved 2 years ago, there aren't that many around!! A good agent, should ensure that all of their staff have viewed your home, so that they know what they're talking about when showing people around, or talking to them on the phone. They should also turn up about 10 minutes early, to ensure that prospective purchasers don't have to wait around, and during that time, if the viewers aren't already there, should check out your home, making sure to put toilet lids down, and lights on where necessary if it's a dark room or a dark day.
The reason they should show the property, unless you're like one previous poster, and have the time to check out with the agent beforehand, what the buyer's are looking for, is that if you're unaware that they're not really keen on gardening, and you have a lovely big garden, you may wax lyrical about how much garden there is, and how long you spend tending it, which could completely put them off. To some people, having a regular bus service could be wonderful, but to others, like myself, it could be off putting, because I used to have a regular double decker bus service running past my old house, and people would look straight in my windows, which I found really intrusive. Of course there are numerous things which you might think are great about your house, but unless you know what the buyers are looking for, you could completely put them off, hence the agent is best placed to do the viewings, and as other posters have said, you pay a lot of money for their service, so make sure you get your money's worth.