Regarding the new AT&T Data Plans for iPhone and iPad
You should probably switch. They aren't perfect, but unless you are an insane bandwidth hog who never uses WiFi, you will end up saving money.
Regarding tethering: if you have light data usage needs while away from WiFi, you should pay the $20 extra for tethering. It's a lot cheaper than $60 for a MiFi.
Personally, I'm switching to the $25 'data pro' plan for my iPhone and my iPad 3G, and the same for my wife which will save me $20/month.
But I'm keeping my Verizon Mifi, because it gives me good coverage when AT&T doesn't. Plus I have a contract, so I sort of have to keep it for another year anyway.