Cricket does indeed have their own towers and network within their coverage areas.
They also partner with Sprint and Verizon for roaming outside of Cricket coverage.
Whether or not roaming is included depends on what market you live in. For example, here in the Chicago market the $50 and $60 plans include 30 and 200 minutes of roaming per month, respectively.
Those plans are $5 off per month right now. Providing that you sign up during the promotion you would lock in that price as long as you kept that calling plan.
I would try them out, I switched recently and have been able to cut my bill nearly in half over US Cellular. I have the $50 ($45) plan.