The S40 is a reliable bike, and a pretty good choice for a first bike to learn on. The bike is light, easy to handle, and uninimidating, and it has been in production for 21 years now. Ones made after 1995 have a 5th gear, while earlier ones have a four speed. Resale on them is pretty low, so used ones are easy to find at a bargain. Parts are easy to find also.
However, the bike is not very refined, does not have a lot of power, and is physically very small, so most people who buy them outgrow them fairly quickly and soon want something larger/ smoother/ faster- when I worked for a Suzuki dealer probably 80% of the people who bought one new were back at the dealer looking to trade it in after one year or less. So I recommend people buy a used one rather than a new one, so they can recover their money when they are ready to move up.
My favorite recommendation for a first bike is the Honda Shadow VLX. As a twin it is smoother, plus it is a physically larger machine that people tend to not outgrow quite as quickly yet still has a nioce low seat hieght. They are also readily available used, and parts and accessories are plentiful.