I never used an ad-hoc network when using Tether-X. Although I don't have broadband in my home, I did purchase an inexpensive WiFi router for $10 (regularly $30, but got it on a killer 24-hour sale), and set it up in my home. This allows me to connect 2 laptops and my printer to the local WiFi network. Then I connect my phone to the WiFi router, turn on Tether-X, and note the IP address. Then I configure that IP and port as my proxy under Internet Settings and I'm done.
Of course, that just works when I'm at home, but I generally don't need it when I'm out and about because then I use public WiFi.
This past weekend, I picked up a wireless bridge/access point/router/client device. Pretty nifty little device, as I can configure it to do just about anything. After quite a bit of tweaking, I was able to set it up for my phone to use the WiFi of the router, then this device will use WiFi under the proxy to get the Tether-X connection, and it goes through a cable to the LAN port a router, which in turn gives full internet access to all devices that access the router.
I've got to admit, it's a lot easier to just turn on Tether-X or Internet Sharing, and have these devices take over for all the devices in the house. Now I am able to use the WiFi printer at the same time as the internet, and couldn't do that before. I would have to disconnect from the wireless internet so I could connect to the wireless printer. Sure is nice to turn my phone's hotspot into something that can be plugged into the WAN port of a router. That means even a device that can't do WiFi can now get internet through an Ethernet cable. Pretty slick.