In every MX we are using a Verizon USB card with an antenna that I've tried to mount as high up as I can go. All of the cards are on a M2M pooled plan so it doesn't cost a ton each month to keep them active. The backups have helped me many times in the past when a circuit has went down for maintenance or network issues. Some sites I ran solely on a cellular card until the fiber circuit was dropped in and ready. I only have one site that needs the antenna mounted higher as the cellular connection is a bit flaky.
It is nice too if a MX flips to cellular, I know it's a circuit issue, but if the MX goes hard down then I know it's a power issue or in one case specifically, the MX kicked the bucket. RIP MX70.