Wow! The Insteon debate continues... I'm sorry I missed the early posts and didn't chime in.
I was an early adopter, and the early gear was, as many have noted, plagued with problems. SH did exchange items purchased DIRECTLY from them, but I still have a drawer with many original devices purchased through a dealer. It was such a hassle to return those I never did complete it for many devices. SH simply wouldn't accept a dealer invoice, period, and I fought and fought on that. So as you've already heard, BUY FROM SH IF YOU'RE GOING TO BUY INSTEON.
As for reliability, it certainly is much better, but it's not perfect. I still replace 1 or 2 switches / year out of about 40. My previous Insteon installation had over 70 devices and the new owner has replaced 2 - 3 per year, so his average has been about the same. It was a tough decision to go with Insteon again after I'd had so many problems, but I did so because the newer equipment was "reliable enough", and because I didn't want to relearn a new system and technology. I'm generally pleased with my current installation, but don't expect Insteon (or possibly any other automated lighting switches) to be as trouble free as a mechanical switch. They go crazy if we are forced to use a generator, and I just lost two to a power surge that took out the whole house suppressor.
Re: turning on multiple swtiches at once... I just tried turning on a local and two remote switches simultaneously (3-gang box). The local was of course fine, and one remote triggered the load switch across the room. But the other remote did NOT trigger that light (in an adjacent room one floor down) and it's normally rock solid. So I'd say that in theory they are supposed to "wait" for each other, but in practice it's not quite that clean. But with rocker switches rather than toggles I don't find we turn them on simultaneously, anyway. A partial second pause in between is sufficient, so in practice probably not a problem.
Finally, re: having control via HomeSeer only... I have to agree with others that the ISY is an excellent idea. I have an ISY, and Elk and HomeSeer. HS has actually become the weak link in the system, and I suspect it always will be. There are not moving parts in the ISY or Elk, and they are much less complex. In fact, I'm seriously wondering if I want to stay with HS long term. They have had many problems with recent betas and versions, so I'll see what HS3 brings. If it's as unstable as the present system I'll likely look for an alternative despite using it for many years.
Good luck with your system, and welcome to the addiction.