One of the primary reasons to jailbreak your iPhone is to use an application called "MyWi" which enables iPhone users to share their cellular data with other wifi devices. For example, if you had a WiFi only iPad, you could use MyWi to browse the internet on your iPad wherever you had cellular service. The MyWi app only cost a one-time fee of about $20 vs, AT&T's monthly fee of $20 IN ADDITION TO your existing data plan. Essentially, users with an unlimited data plan from AT&T really only had unlimited data as long as you only used it how AT&T wanted you to.
In March of 2011, I saw two articles which convinced me to give AT&T's tethering plan a try:
Users reported that iPad's connected via a tethered iPhone would receive GPS information as well
AT&T was sending threatening letters that users of the MyWi app would automatically be enrolled in their Personal Hotspot tethering service.
I was really keen on seeing how the iPad could act as a large screen GPS app while driving while being fed location information from my iPhone and I was really NOT keen on AT&T "automatically" enrolling me in their Personal Hotspot service (yet, ironically, their scare tactic worked).
Anyway, for my first test, I enrolled in the AT&T personal hotspot service, set up a password I would use to let other devices connect to my iPhone, and then tethered my iPad to the phone. I then loaded the Google Maps app on both devices and watched to see how well the GPS tracking worked on the Wifi only iPad during a drive home -- the answer was, not very well. While the location information was being updated on the iPad, it wasn't nearly as real-time, nor as accurate as the iPhone tracking. It appeared that the iPad was still using very basic triangulation to establish GPS, so more often than not, my location was mapped onto buildings rather than on the streets. Also, the location only seemed to be updated about once per minute, so it was not getting real-time updates during the trip. Although the large map display was VERY COOL to have in the car, it wasn't nearly accurate enough to use as a turn-by-turn GPS replacement. Lesson learned, I'll probably spring for the 3G iPad on my next upgrade solely for the enhanced GPS functionality.
Other than that, everything else about the AT&T Personal Hotspot was very favorable. Download speeds were better than I had expected and the iPhone battery life seemed to last longer than when I used the MyWi app for tethering. Also, connecting to the iPhone from the iPad was very fast and easy, and if I was a regular business traveller, I could easily justify the extra expense.
For now, I can still add and remove the Personal Hotspot service on a month-by-month basis, so I'll cancel the service at the end of this month. But for any upcoming trips, I would quickly reactivate the service.