It's basically supposed to be sweet. Some types, when really ripe contains some sort of alcohol (?) so eating too much can make you dizzy. If you want to try it, look for the Thai durian (montong). Unlike some types, montong has a very thick flesh and very small seed....... must warn you though, montong is very big. Oh, not recommended for people with cholesterol and digestive problems.
Jackfruit is a totally different kind of fruit. It contains a lot of sticky sap (?). Opening it can be quite a challenge, specially if it's not ripe enough. It's also sweet but not mushy (?) like durians. Not recommended for people with digestive problems and asthma.