I am generally using my iPad (WiFi only) to tether to the iPhone. Occasionally I will tether the HP Laptop to the phone.
MiWi is more expensive, but, in my experience, it has not been unreliable. They also have an add-on app called MyWi OnDemand, which works through Bluetooth, and seems very easy to work with. There is, however, some bit of difficulty in getting a connection to the Laptop occasionally by WiFi when using MiWi. I'm not sure why, but the builtin tethering works better when connecting with the laptop. The iPad - no problem either way.
The only other tethering I have done (do currently also) is Verizon's paid tethering plan. I don't mind spenting the extra bucks, but generally need a bit more than the allowed 2 GB in a month... not much more, but some. I usually split the tethering between the paid plan and MiWi. In this way, I am trying to be as ethical as possible with my payment for the 2 Gb and tethering allowance in general, and at the same time making the best use of my unlimited bytes when I need some more. In case you might not know, Verizon has two pools of data from which the bytes are drawn. Or to put it another way, two different paths of internet access, and (in my case, using the iPhone) the one is used by the device strictly for tethering... the other communicating through the "unlimited data plan" path that the device uses to get Internet directly to itsself. Hope that makes sense.
Brian - Sent from my iPad using iPF