Don't sweat it, Mike. Go with your gut feeling about the people you're dealing with. Definitely read the contract and negotiate the best deal that fits your needs. If the dealer is ADT corporate, don't worry. You're probably ok. It they're an "authorized dealer" be very careful. Some are honest. Many are not.
Don't let the ADT bashers worry you. Many of these guys have been trying for years to compete with ADT. Because their own businesses are not particularly successful they blame ADT instead of evaluating their own shortcomings and finding ways to improve themselves.
I used to sell against guys like these all the time before I sold my alarm company. We were small and we ran a tiny, non-UL listed central station out of offices attached to our home (previously a doctor's home / office). The local bad-mouth artists would say our c-station was in the basement or the garage. They'd claim we sold inferior hardware because we were small. One guy used to tell customers he knew me from the local high school and he would lay down a litany of lies, ignoring the fact that we'd never met and that I grew up in a different state. I beat those characters at sales every day of the week by offering more protection, using top-of-the-line systems and doing a professional presentation. Frequently customers would ask me what I thought of one or another competitor, especially the one I knew were bad-mouth artists. I'd just say, "He's a competitor. I prefer to let him stand or fall on his own merits rather than speak good or ill of him." Then I'd get back to my presentation. If the customer said that the other guy had spoken ill of me I'd just say, "It's unfortunate some folks resort to that sort of thing to make a sale."
What you've seen here is the same sort of thing. A few sales people or small dealers who have a hard time competing with ADT bad-mouth them rather than offering any constructive advice.
It's not all black and white, of course. A couple of these guys have offered good suggestions about contract term, price negotiation and so on. Good for them and better for you.
Oh, by the way, I have nothing to gain or lose in your choice either way since I don't offer installed systems. If you were interested in doing it yourself I'd offer my services. However, there is something I can do for you and it won't cost you a cent. Ask the rep to list exactly what make and model sensors he's putting in. Then go to my website. I list over 41,000 products, including many of the most popular brands of alarm systems. Compare what you're paying to the retail price of the hardware. That will give you a way to compare the equipment value between competing installers.
Best of luck.