Setting up Personal Hotspot from that point is dead simple. You tap on Settings > Personal Hotspot, enter a Wi-Fi password that up to two other people will use to share your 3G data connection and then tap done. Flip the Personal Hotspot button to On, and you have an instant Wi-Fi network.
From another device with Wi-Fi connectivity, you'll see the network appear and can connect to it by entering the password. I've successfully attached to the hotspot from another iPhone, an iPad, my MacBook Air and a friend's Windows 7 PC (not all at the same time, of course). You can tell when someone else has connected to your phone's data connection, as the status bar at the top of your iPhone screen turns blue and pulsates (see below).
It also adds a line telling you how many people are currently connected to your Personal Hotspot. At that point, you can shut off the screen and just use the phone as a wireless modem -- no need to burn even more power with having the screen on. When you're connected from another iOS device, the standard Wi-Fi signal strength icon on that iPhone/iPod touch or iPad is replaced with an icon that looks like a pair of interlocked rings.
I used a Wi-Fi link between devices, but you can also enable Bluetooth or USB tethering. I could see where using USB would be useful in those situations where you need to charge your iPhone at the same time you're using the Personal Hotspot. While using the Personal Hotspot, I've been keeping my iPhone powered via the AC adapter to make sure the battery is topped off.
So, how fast was the Personal Hotspot connection? Using the free Speedtest.net app I was consistently able to get download speeds in the range of 1.3–1.7 Mbps, and upload speeds were steady at about 1 Mbps. I thought that was slow until I tested the free hotel bar Wi-Fi service with the same app. Download speeds over the shared hotel Wi-Fi ranged from a low of 0.08 Mbps to .89 Mbps, while uploads were in the area of .37–.92 Mbps. Getting the picture? The 3G connection through the Personal Hotspot was actually preferable to the Wi-Fi being shared by a handful of other users. I didn't test the Wi-Fi access speeds in my hotel room, as I really didn't want to spend the money to try it out. I'd be willing to bet, however, that they're close to what I saw in the hotel bar and limited by the hotel's landline connection.
In terms of real-world usage, I'm using the Personal Hotspot connection primarily to grab email and upload updates of my book to my publisher. File sizes range from absurdly small to about the 50 MB range, and the service (while nowhere close to what I have at home) is very usable.
Sure, the cost is an issue. I'm spending $45 a month more on my phone bill for a measly 4 GB of file transfers. But when I'm traveling, I'm usually not using that much bandwidth anyway. In this specific case, I'd be spending $44 in Wi-Fi fees over four days for the far worse speeds on the hotel wireless, so the cost is a wash. For me, having the flexibility to connect my iPad or MacBook Air to the internet from anywhere there's a 3G signal is well worth the cost.
Any questions or personal comments? Leave 'em in the comments section below.