Essentially, I pointed newcomers to the main article here on 3G/WiFi and WiFi-Only posted Saturday, which covers the basics and I mentioned one aspect I consider important:
People realize they can use WiFi access at home, and if they're not using the Kindle outside the home much, that would be enough. And it should be, although some forum participants cautioned that their WiFi networks can go down at times.
But after reading a few notes, I realized that there are some who actually don't have a WiFi network in the home and may not realize they'd need to set one up or that if they do leave the home regularly, the ability to download a book while here or there will no longer be available the way it was with the Kindles 1 and 2 unless they are in a WiFi hotspot and can access the network easily (whether needing a passkey or a fee, as only some hotspots are free).
The less-expensive Kindle 3 (or UK Kindle 3) comes with WiFi only -- but I am guessing there are some who actually do not have a WiFi network set up in their homes and might not care to have this done, in which case that wouldn't work.
If people know they could access any printers in their home that recognize WiFi networks, without needing cables, that might be an added incentive to spend the ~$40 - $50 to buy the router needed.
If you do have a WiFi network set up at home, it would allow you to just click on a Kindlestore book on your Kindle to get it TO your Kindle w/o needing to hook up the Kindle to a computer at all and, often, it might be faster to download books or do a web lookup than with 3G cellular network access.
But if you don't have a WiFi network at home and are not likely to have one set up, what I just described won't work and you would have to download an e-book to your computer first and then transfer it to your Kindle with the USB cord.
In that case (no WiFi network at home), if you do have AT&T 3G cellphone type coverage in your area, you'd be better off with the 3G/Wifi model or UK 3G/WiFi model as you could then just use that 3G cellphone wireless capability to download a book direct to your Kindle without needing your computer or the missing WiFi network. And you could still use (automatically) the WiFi capabilities of any place you visit that does have a WiFi network setup.
The best of both worlds.