I have owned two Harmony 676 remotes, and I'm going to buy a third one today. I haven't had any problems remotely like the ones mentioned in this thread. I have several separate A/V set ups in my house, thus the need for the multiple remotes. Also, having all the remotes being the same type make things easier on the mental workload.
The Harmony website will work fine with FireFox, they just show that stupid message. I can also put in my vote that the web site is a lot better than the software, which actually just looks like a version of the website in a separate little window.
While there are definately flaws with the devices, and I admittedly don't have much experience with other types of remotes, I really like the Harmony 676.