I have a Cavalier King Charles Spaniel!!
I got him at 8 weeks, and he recently turned a year old. He has a very sweet disposition, loves everyone, gets along with other dogs (we're at the dog park every day), and has been easy to train. My only "problem" is that he goes nuts when he sees a cat, barking, trying to get at it, etc. BUT the funny thing is, there have been some cats that just didn't run away from him, and when he gets right up to them, he just stops and looks at them eye to eye.
I'm always getting compliments on what a beautiful dog he is, with the feathered tail and legs. His tail is always wagging.
Housebreaking...well, I'm a member of a Cavalier forum (cavaliertalk.com), and it seems that Brian fits the breed trait of taking a bit longer to housebreak/being particular where they potty. He will not potty on the concrete sidewalk, which is hard living in a city with not too much grass around. But we do have a tiny tiny patch of grass and that's where he goes.
This breed loves being around people and will follow you everywhere you go, even in the bathroom.
A few people mentioned the neurological issues that many of them have, but in the past few years, breeders have been trying to "breed out" those genetic issues by only breeding the ones that have been tested negative. The website I posted has a TON of information about this, please take a look.
You don't see many CKCSs around, so people will be asking you "is that a King Charles?" , because many have never seen one in person.
I'll post some pics; I'm on DHs laptop so I have to get onto my own to get the pics.
I'm VERY happy with my decision to get a King Charles!