We have been using Millenicom since last May. The first device they sent me locked up a couple times, then failed altogether. Their customer support is superb. They sent me a new, much better device, a Pantech Jetpack MHS291LVW. It has never failed, and it displays your current usage during the billing cycle.
We hit 30 GB one month and a tiny bit over 20 GB twice, and they haven't complained. The new device lets me track our usage more carefully so I don't run the risk of getting penalized for an overage.
Regarding placing the service on hold, I asked them, and here's the response I got:
You can cancel the service for now or put it on vacation hold.
Vacation hold can be for up to 3 months in any 12 month period with no
charges while on vacation and no charge to take it off vacation. You
can resume service any time during the 3 month period by emailing us
at billing@millenicom.com. The vacation hold option is only cost
effective for you when on hold in one month increments as you are
required to pay for the entire calendar month if the service is active
for any part of the month.
If you chose to cancel the account it can be reopened at any time with
a $19.99 activation fee (reduced from our normal $49.99 charge).