A GPS itself wouldn't be particularly useful in dear old Singapore, you'd probably recognize the place faster than you can dig up some GPS coordinates and look up the location (or plot it to a map overlay).
Anyhow, a GPS is also of fairly limited use in built-up Singapore, since signals are fairly weak - you do need to get out in the open to get any sort of accuracy, and even in HDB estates, the reception isn't fantastic.
If you are a frequent globetrekker, and have gone to more places than you care to remember, perhaps you might deem the need for one.
There are many uses of GPS, it is not only use for navigation only as most people perceived. I use GPS to take down and measure my time/distance/speed and log them for my running, hiking and other sports. (sorry for the couched potatoes!) I dun use the navigation feature much in Singapore. Or if you want, you may acquire Geocaching as a hobby (I dun bcos I find it boliao).
Lastly, people say you'll never get lost with a GPS on hand, but I still prefer my good old Silva compass and map ;p :blah: