Have had the 550 for about a month now, and overall, I like it. But I've experienced a lockup with it twice now where I've had to remove a battery and re-insert it to reboot the remote. Any thoughts? (aside from calling the abysmal Logitech CS)

I also have the exact same problem with my 550 and would love to hear if anyone has found a solution to this problem

I had a problem where my remote would lock up sporadically but it sounds a little different than your problem. I assumed it was because the remote didn't like the rechargeable batteries I was using, however this last time changing the batteries didn't help. It turns out that one of the rubber buttons was getting stuck down, which prevented any of the other buttons from working. It wasn't obvious either. I didn't find it until I literally tried to press every single button one by one. When I got to the pause button I realized that it was just slightly hung up on the faceplate. Once I got it unstuck everything worked great. Sounds like you may have a different issue, if replacing the battery works every time, but it might be worth a shot.

I have been experiencing the same issue with my Harmony for Xbox (essentially the same model). Spent some time with Harmony support the other day and have not had a repeat occurrence as of yet. What we did was reloaded the firmware in "safe mode".

1- remove batteries
2- plug in to computer while holding the power button
3- check for "Safe Mode" on screen.
4- reload firmware
5- reload your set-up
6- unplug remote
7- hold the "2" button while replacing batteries.

You may want to call harmony and have them walk you through this as I am just going from memory on this.