Ok so I went ahead and bought the service. Here's what I found out: You'll have to keep the tether client application running on your Mac or PC. So not gonna work for wifi-only iPads. Then you will need to keep the html5 webpage open in order to receive data. They instruct you to set iPhone's screen lock to "never" so it doesn't automatically shuts the web page down in 1 minute or so. So I imagine you can't take phone calls when tethering as the webpage will go offline.

Pricing wise, the first year costs $15 and will be $30 from second year on. They set my paypal up for recurring payment.

Performance wise it's pretty fast on my iPhone 4. Setting up took about 2~3 min, very easy to do. The html5 app needs to connect to both the tether website and the client computer. Took me a few tries to connect to both but once they are connected the speed is pretty comparable to tetherme in jailbroken iPhones.