It's a useful tool and the service check form that you can integrate into your website is a great feature.
The coverage maps will always only be estimations of coverage, not taking true foliage or building obstruction into account.
My only real gripe with towercoverage is that they nickle and dime anything you want to export from the site. Want a google earth kmz file, that's $$$. Want a 477 report, that's $$$, etc. I wouldn't mind so much except we are already paying them $50/month to essentially host a form and coverage map.