Cell coverage maps represent the best case scenario for each carrier and likely won't reflect the actual coverage that you get with your phone. Coverage relies on many factors, including terrain and obstructions (i.e. mountains, trees, buildings), your plan (postpaid plans have better coverage than prepaid and will often include roaming), weather, cell towers themselves (if they are down for maintenance) and your equipment.
Equipment plays a big part in your coverage. Owners of older phones that only have 2g and 3g will find their coverage decrease over time as these networks are phased out and towers converted to 4g and above (for more information research "sunset 2g" and "sunset 3g"). Newer 4g phones have better coverage, but not all 4g phones are created equal. Different carriers use different frequencies and bands, and as a result, even though a carrier's coverage map will imply they provide coverage for a specific area (for example, I-95 between Orono and Houlton), that coverage may be based on cellular frequencies that your phone can't access.
Newer unlocked cell phones often have more cell bands than carrier phones. There are a number of websites that provide cell phone specifications for comparison purposes.
It is possible to take advantage of low cost prepaid plans to supplement your regular cell plan and create overlapping fields of coverage. Verizon and US Cellular have the broadest coverage throughout Maine, but there are certain areas where AT&T (Acadia) and T-Mobile (Sugarloaf and parts of the Canadian border) have strong coverage. If you have a few older, unlocked phones, you can purchase cheap plans for supplemental use. My primary carrier is Verizon, but I also have a $3/mo T-Mobile prepaid number and $10/quarter H2O Wireless (AT&T network). This has come in handy in Acadia where AT&T has strong coverage and Verizon's coverage is spotty.