The short answer is, absolutely.
An SSL certificate provides security for your customers data. Without one, their data is not protected and you will, most likely, lose customers. That is because even little old grandmothers surfing the web know that they are supposed to look for the https in the url or the image of the lock when placing orders. When they don't see either, they may go to another site. We provide two options for SSL's: shared and private. As far as security, they are the same.
The main differences are that the shared SSL is free but you cannot control the name in the url. The private SSL costs around $50/year plus a $2/month charge for the required private IP (that cost varies with the host) but you can control the name in the url. Some shop owners don't like to have the shared name show up in their url's while others don't mind. Using a shared SSL may not work with some payment modules but there are only a few of those. Otherwise it is fine to use.