The latest MyWi works very, very well. If you don't have WiFi turned on already, then there's a hiccup while it "cleans up MyWi" [sic] to enable Wifi, and then you have to enable it again.
And of course the USB and Bluetooth operability on a jailbroken iPad or iPhone isn't going work perfectly if you're trying to hook up to a Mac.
But the WiFi capability more than makes up for that. I wish that Bluetooth worked easily, because of the low battery consumption, but most of us are going to know to be connected to a power supply when we're using MyWi.
If you have an iPad and an iPhone, and you're paying for data on both of them, there's a new addition to Cydia called MyWi On Demand. It would effectively allow you to drop the data plan on one device or the other, because you'd be able to seamlessly tether to the other device, automatically and quickly.