It is worth if you have a super fast DSL connection or a local network where you transfer files quite frequently. Most of the Broadband connections are not fast enough to get the full benefit of "n". In the future this might change though.
Erm, I don't believe that any broadband connection is going to be fast enough to notice the difference. n vs g is not going to affect your WAN speed, but will affect the speed of your LAN.
If you have another computer hardwired to your router, or a computer (or other network appliance) that supports N speeds, then it is worth upgrading if you anticipate transfering files between machines on a regular basis, or streaming video (I rarely have trouble streaming video over G from a computer upstairs to my TV downstairs by way of a Philips Streamium SL-300i, but the video is standard definition and either mpeg2 or divx encoded -- if you want to stream hi-def or DVD quality VOB files, then you'll need N).
If you have one computer connecting wirelessly to your access point which is in turn hardwired to a DSL/Cable connection, then you're unlikely to notice any difference in G vs N. The only exception to that would be signal strength. If you're not getting a full connection to your wireless computer, then upgrading to N could help your speed because a stronger signal will usually give you a faster connection.