I have four Panasonic KX-TCD450 and have been using them for around 5 years.They are great and have lots of features, some of them are excessive such as walkie-talkie which I never use. The useful stuff for me is that you can send the caller from one handset to another one if he wants to talk to a different person in your house, you can create a conversation by adding one more handset to the call and you can use louder-speaker. You can also make an internal call from one handset to find another one
But my model has small disadvantage is that you cannot hold the handset easily with your shoulder when you do something at the kitchen because of the slightly inconvenient rounded form of the handset which tends to fall off after. But the newer 550 and 700+ handsets look thinner and have even more features and probably they don't have such a disadvantage. So I would buy the newer ones.