I was an installer for years. In my experience ADT is reputable, however I heard their charges can be out of this world. I have worked with ademco, napco, dsc, moose, ge, fbi and more. The best system I have worked with is the GE. It has ease of operation, and it is easy to program, plus it can handle wireless devices for those tough to wire locations. So it can handle wired devices as well.
As far as servicing them, They carry the GE name, so they are reliable. The only issue is who you get to install it. I recomend researching your dealers, maybe a testimonial or two. Or you can go the estimate route. Look for coverage. You want as much coverage you can get for your dollars. Do they recomend barrier bars for your basement windows or a motion detector. The latter being the potentially more costly. But if you are in the basement doing laundry one night the motion detector would have to be shut off. Whereas barrier bars can be left on and you maintain the integrity of your protection. I hope this has helped you.