Unfortunately, "tethering" with AT&T is risky business. If you use a noticible amount of data, they can (and HAVE, in some cases) charge you PER KB USED EXACTLY.
I, myself, use an application on the iPhone called "MyWi" for jailbroken devices, which allows me to broadcast a Wi•Fi hotspot from my iPhone. I use this for getting online with my XBOX, and surfing the web with my laptop, but NOT to download anything. That would be extremely noticible.
I guess if you're good with moderation, you should be fine.
However, if you're having trouble with the 5GB limit, I can tell you right now - if you think you're going to use MORE than 5GB a month while tethering, and still fly under the radar, you're insane.
As far as your friend connecting her iPhone to your CPU and having it not work - did you actually have the newest version of iTunes? The drivers that allow tethering come with that. And in my experience, I've had the best luck with MyWi broadcasting a Wi•Fi hotspot, BECAUSE of it's simplicity. Use your laptop to connect to your Wi•Fi network, and that's it. Simple as hell.
Overall, if after everything I've said, you still think that it sounds right for you, go for it. I'd recommend sn iPhone to everyone I know, it's THAT good. (Once it's jailbroken)