I recently upgraded my Mi-Fi. I now have the Jetpack 5510L. This is a big improvement over the Novatel 4510L it replaced.
I'll keep this short review quick and simple. This is a big improvement over the older MiFis I've been using. It is rated at 8 hours versus 3 or 4 hours I was getting with the older device. Also, it has a nice read out display with navigable menus. Lastly, the thing I like about it is the the ability to charge and use the device with USB. The last model had a weird set-up that prevented charging when you plugged it into a computer; making it act as a data modem. This model, you can switch how you want USB to be handled via the web admin interface, I also like the handy and quick glance of the monthly data usage in the LED panel.
Overall, I think it is a big improvement over the previous models.