Just got my Karma hotspot and I have to say I'm impressed with the coverage so far. Though the 4G network can be spotty at times especially when moving, this is a perfect solution when I need quick Internet access on my laptop for work when traveling. Also great for my kids wifi-only iPads and iTouches during those times when parents need a break.
Another cool feature of this hotspot is the ability to securily share the connection with complete strangers. When they see your wifi hotspot, they can connect and receive 100 megs of free data to use and you'll also get 100 Meg's of free data credited to your account.
Data never expires and there's no contract. Use what you pay for.