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.
