The new version brings much improved performance, a new "ncat" command line tool for moving data around over the network (intended to replace the much-used netcat tool), and ndiff, a tool for comparing the results of nmap scans over time (for example, to see what new ports and services are running).
Although primarily a command line tool, nmap has a companion GUI, zenmap, that makes it easier to use for those less familiar with its wide range of options.
If you're already an nmap user you'll probably want to upgrade. If not, now is a good time to have a look at one of the most useful security tools around. Nmap is free and open source, and supported on a wide range of operating systems. Corporate users should check with their security staff before playing with nmap, as it may set off antivirus software as a "potentially unwanted" application or similar.
See http://nmap.org/5/ for details