I use a Huawei E303 in a Raspberry Pi Model B (not sure if its the newer or older revision, but I bought it in September 2013). First I struggled quite a bit, since it worked reliably when hotplugged but not at boot-time. Then I found the post in this thread http://www.raspberrypi.org/phpBB3/viewtopic.php?t=32307&p=439933 about creating /etc/udev/rules.d/41-usb_modeswitch.rules which fixed the problem.

Now, everything is fine at boot-time and/or when hotplugged, but when I leave the Pi running then after anything between a few hours and 3 or 4 days, the interface just seems to vanish.

I thought it might be to do with power so, as suggested, I plugged the dongle into a powered USB hub, rather than the Pi directly. But it's still not solving the issue. Do you have any pointers for me on that? Any help would be much appreciated!