I'm a big fan of Panasonic. Their products are well designed and well built. That's probably why the KX-TG5100M 5.8 GHz phone system was a disappointment.  
 
First problem: Sound quality. Memory chips are inexpensive, but Panasonic saved money by putting a small memory chip in this system. Compression technology is utilized to fit 15 minutes of recorded messages onto the chip, and you can tell -- sound quality is AWFUL. Ironically, the best sound quality comes from the old analog answering machines that still use cassette tapes. (So much for technological advance.)  
 
Second problem: The power pack consists of three cheap NiCd AA batteries encased in plastic. Even worse, they're very low capacity NiCds -- just 850 mAh. (NiCd is the old technology for rechargeable batteries. Cadmium is a nasty environmental toxin. NiCd batteries drain quickly, even when not in use, and they have a "memory effect": If you don't drain the power completely before a recharge, they'll only charge up to the current level on the next charge.) The phone handset only works a few hours before recharging is needed, and storing it on the charger base will significantly reduce the life of the batteries. If Panasonic had opted for the current technology of 2200-mAh NmH (nickel metal hydride) batteries, the 4-hr usage time could have been boosted to 10 hrs or more, with no memory effect. Enough cost cutting, Panasonic. If you're going to charge people so much for a product, build it with quality components.  
 
Third problem: You have to endure a goofy voice that tells you things that could be more efficiently conveyed through visual indicators. I suppose an electronic voice is Panasonic's way of adding a "Gee-wow" factor, but it's another example of how new technology makes our lives less efficient. Older answering machines have an LCD display that tells you how many new messages you have. This machine has just a tiny red light, so if you call-screen a message and don't erase it, you have no idea whether other messages have been added (unless you start pushing buttons). You have to waste time pressing buttons and listening to the voice every time. Also, you have to press extra buttons like Mailbox 1 every time, even though you never use other message "mailboxes" (used for password-protected messages that can be storage for other users of the system).  
 
More gripes: The antennas on the base unit are five inches high. They get in the way. Also, you can't hear your own voice in the handset, as you can with other phones. It takes a while to get used to that. Also, the handset is fairly bulky and heavy.  
 
On the plus side, the range of a 5.8 GHz phone is fairly good (although quality does degrade with distance). I still worry about zapping my brain with high-energy radio signals, but going cordless is nice.  
 
Wish list: In addition to fixing the above-mentioned gripes, it would be great if there were a way to speed through boring messages. My old digital answering machine allowed me to increase the talking speed 1.3x and 1.6x, in addition to jumping back in 5-second intervals and slowing down the recorded message (all without changing the pitch, and all controlled by a simple, intuitive jog dial). The Panasonic does allow you to slow down the message playback, but that's it. 
