My Novatel 4620L had also been working well since March. I had some problem connecting in Canon City, CO last month, but forcing the unit into EVDO only helped.
However, on 1 Aug it quit connecting. I'm back in New Orleans, it was working ok, then quit connecting. I took it into town for stronger signal, but no go. I thought maybe my 6 months was up, although that was suppose to happen on 1 Sep. So I tried to add $5, when I called was notified that number no longer valid.
Was told this happens occasionally. I am having it reflashed for another 6 months and will see how it goes.
I can say the low monthly cost so far has been worth it.