My experience, happily, mirrors that of others here.
I called on a Friday afternoon, when our Noritz tankless water heater was intermittently failing to fire up, and another company claimed that we needed to replace everything at enormous expense. I wanted, at the very least, to get a second opinion.
When I called, the person who answered the phone first asked me, in a concerned tone of voice, are you completely without hot water? I reassured him that it wasn't a dire emergency, at which point he put me on the schedule for Monday (the next business day). I got the impression that, if it were more urgent, he would have found a way to get someone to come that same afternoon or during the weekend.
Ryan showed up on Monday morning, apologizing for being about an hour earlier than expected. He took a look at the heater, and quickly explained that the situation was nowhere near as catastrophic as the other plumber had assumed. It turns out that the supposed corrosion in the heat exchanger, observed by the other plumber, was entirely normal for a unit of this age and was by no means a sign that it was at the end of its life. The ignition problems were simply due to poor wire connections in the remote control - something that the other plumber had explicitly denied when I asked about it. Ryan had the whole thing fixed in ten minutes. The total repair price was about 2% of the water heater replacement that the other company was pushing.
Water Heater Pros could have done like the other company, and tried to get me to pay a fortune for an unnecessary replacement. Instead, they chose to be honest and to recommend a simple and effective repair. It's such a relief that there are contractors like this. And, by the way, Ryan was absolutely delightful.