As with everything else, it depends on your application (what you’re going to use it for). If it is for moving around the house and using your notebook to access the internet, you probably won’t notice any improvement in speed. The older “a” and “g” standards provided up to 54Mbs in bandwidth on either the 5Ghz or 2.4Ghz bands, so unless you’ve got a fiber pipe directly into your service provider, your speed will be limited by your ISP. If you only get a 3Mbs stream from AT&T, you’ll only get 3Mbs through your wireless connection. That’s simple physics.
On the other hand, many of the 802.11n routers allow transmission on both the 2.4Ghz and 5Ghz frequencies, so you could toggle between the two until you find the cleaner frequency at your location. Also, the new n standard promises to double the range of coverage, so you might be able to finally sit on your deck while you watch the latest youtube folderol.
However, you could achieve a greater distance more cost-effectively with a higher-powered antenna hooked to your router. Though if you are in the habit of using your wireless device at “Wi-Fi Hotspots”, your n adapter should allow you greater roaming flexibility. In this case, it should be worth it.