A live chat feature can be very beneficial, especially if people have questions and don't want to have to wait for an email reply. If you are willing to dedicate the time to put it to use then it can help improve your sales. I have used them in the past and loved it.
I love when I come across websites that have a live chat feature. If you are going to do it though make sure that you are going to be able to cover a good timeframe and not be offline all the time. That can be very frustrating if people come to your website and are hoping to have someone to talk too.
We use Crafty Syntax Live Chat. Great software that is easy to install, setup and use.
It is open-sorce, so the cost is nothing. (doantions to the creator help defray his cost, but are not required)
you can find out more about the software here: http://www.craftysyntax.com/
Live chat improve your customer service. It is very useful when the customer has questions and they can get your reply very quickly. I think the customers like the websites which offers the better service.