The internal GPS in the cellular model seems to be quite adequate for most pilots (even without a SIM), but officially if you lose your fix and are away from any internet connection you could see a delay of up to 12 minutes as the chip tries to find itself again.
This is quite rare, but still a concern. Many of my (SkyDemon) customers go out and get an additional Bluetooth device from companies whose core business is GPS, like the Garmin Glo for instance, or the GNS2000. An alternative is to have a backup device like a phone which could run your nav software or might even be able to generate a 3G wifi bubble at altitude...