AirPort Extreme features
A USB port has been present on the AirPort Extreme for quite awhile, originally intended to make any USB printer available across the network. At a time when network printers were expensive, this alone was a great feature. Wireless print servers also used to cost around $100, which made this feature well worth the extra cost. They are now available for less, but most routers still do not include this feature. The AirPort Extreme's USB port can also share a USB hard drive across a network. Some people have even connected a hub to it and connected several USB devices, but I haven't had any first-hand experience with that.
Many Wi-Fi routers now offer "802.11n" speeds, but with an important caveat: if you connect slower devices (i.e. 802.11g or 802.11b) to your "mixed" 802.11n network, it will degrade performance. Also, to get the most benefit from the 802.11n networks, many people prefer to set up 802.11n networks using the "5.0 GHz" frequency, which is less crowded as the 2.4Ghz frequency, but is also used by many cordless phones, baby monitors, and other common devices. Unfortunately, even some new devices (such as iPhones!) can't access 5.0Ghz 802.11n networks. Given the fact that most people have a mix of devices, most will setup "mixed" 802.11n networks on the 2.4 GHz frequency, meaning that you often won't get very good 802.11n speeds.
Newer AirPort Extreme units offer a solution to this problem by allowing you to setup a "simultaneous dual-band network" which, as you might have guessed, let's you create one network for your older pre-802.11.n devices, or devices which can't access 5.0GHZ and create a separate "pure" 802.11n 5.0Ghz network. I did that at home and suddenly streaming to my AppleTV worked much better and more reliably, as did transferring large files between computers. That is not a feature that you will find on most other routers. If you live somewhere with a lot of Wi-Fi units around you (such as an apartment building), you may find that dramatically improves your Wi-Fi network's performance.
Another nice feature on the newest AirPort Extreme units is the ability to create a "guest" network. This will allow people to get online, without having to give them access to your main network. If you're one of those folks who are willing to share your Internet connection with your neighbors, but want to keep them from being able to access your local network, this is a good feature for you. Or setup a separate password-protected network for friends to use when they come over.
A feature which many people might not use (but which was important to me), is the wired Gigabit Ethernet connection that the AirPort offers. If you connect your computers to the AirPort using Ethernet cables, you can achieve much higher data transfer speeds across your local network.
There are some other "power user" features such as DHCP reservations (a method to always assign the same IP address to the same computer when it connects to the network) which can be crucial in some situations where port-forwarding is being used. You can also give timed control access to the network using MAC addresses. Want to make sure your kids aren't spending all night online? Have the AirPort limit the hours they are able to access the Wi-Fi network. (No, it's not foolproof, but for most people it will be sufficient.)
The AirPort Extreme is not without some potential drawbacks. The first is that it only includes three LAN ports, while most other routers include 4. This is a definite improvement over the original AirPort Extremes, which only included one. I consider this a "potential" drawback, because in reality I suspect that most people will not need more than three.
A slightly more annoying shortcoming is that there is no web interface to the AirPort Extreme. All configuration is done via AirPort Utility, which means that you can't check the status or any settings using your iPhone or iPad. That's a minor annoyance, but one that I have run into on occasion.
One important note is that the unit does tend to run warm or even hot, so make that that you give it proper ventilation. I would recommend that you avoid putting anything on top or or underneath it, and make sure it rests on a hard surface.
About the only features that I have ever missed on the AirPort are the ability to prioritize traffic and the ability to block access to certain websites or protocols. If you are looking to, for example, block the use of bittorrent on your network, the AirPort Extreme will not help you. Nor will it help you make sure that your VoIP traffic gets highest priority.
For me, it's worth it
I buy Apple's Wi-Fi hardware for the same reason that I buy its computer hardware: it works better than many competitors' products, has been more reliable, and doesn't require me to spend a lot of time maintaining it. Yes, you can find a cheaper router, just like you could buy a cheaper laptop than the MacBook Air I'm using to write this article.
I'm sure that many people who use cheaper routers find that they work fine for their needs, just like I am sure there are some people who are very happy with the $400 laptop deal they found in the Sunday paper. But I'm also sure that there are many people who struggle with frustrating issues related to their Wi-Fi networks and never realize that they might have a much better experience with better hardware.
Image via Apple.com.