I found it really easy but I've built many computers over the years so it wasnt a daunting task. I've also taken apart Apple and other notebooks previously for these sorts of upgrades. I recently mentioned in another thread about Hard Disk upgrades that it took about 10 minutes to take the computer apart and put back together.
I used the guides from ifixit.com which were very straight forward and easy to follow. I ran in to no problems taking apart my notebook. All the screws were easy, the wiring was simple.
What I will mention though is the Antenna cable for the Airport card has a groove cut out for it on the Speaker, make sure you put that cable back exactly how it was within this grove or you may have difficulty in putting the top of the notebook back down.
When I looked online to get the card it was like £70 from ifixit and other stores. I got it from ebay for £15 and its worked great ever since I got it. Even came with a Driver CD for Windows. (It worked straight away in Leopard upon first boot up without me doing anything).
One of the best upgrades I ever did btw. When copying files or performing Time Machine backups the highest throughput I ever got on 802.11g was around 3.8MB/ps sometimes hitting 4.1MB/ps (Rare) with the n upgrade I regularly hit 8MB/ps to 11MB/ps - This is when copying files to my Server via my Airport Extreme. The server is connected to the router over Gigabit Ethernet. So quite a nice upgrade and very affordable.