I was on Ting MVNO cellular for a few years, but left, mainly because they never offered support for Apple Watch cellular plans.
Migrated to ConsumerCellular, which really caters to the retiree crowd, but it has Apple Watch plans for $5/month/watch, which is the cheapest out there I found.
Overall, I think my bill was a little lower on Ting than CC, but I really like having the backup of cellular on Apple Watch, so it was worth a little more $.