I think you will find that no matter what part of a Harley you ask about you will get some positive answers and some negative answers. Harley quality isn't exactly world class, so the amount of variability is significant. In other words, just because ten other guys say that their grips have always worked great doesn't mean you won't get ten other guys chiming in tomorrow that their grips are junk. One of the things I've noted over the years is that Harley is very slow to react to quality problems, but in some cases they eventually improve some parts after the quality gets bad enough to impact profits due to high warranty costs. If the stuff fails after the warranty then it seems Harley looks at that as a good thing, since they get to sell you more parts to replace the bad ones.
If you are worried that you will continue to have problems and don't want to keep paying forever, you could look at the alternatives on the market. If you really want heated grips, unlike the situation with some other parts you aren't limited to just what Harley sells. Do a search for Heat Demons for instance.
Jerry