I bought a Waterpik Cordless WP-450 for myself a couple of weeks ago from the large chemist chain (cheaper on Amazon). I fully charged the unit as per instructions - and the first time I used it I could not believe how clean my mouth felt. All food bits came away easily, and although I did get a little 'wet' the first few times I used it, I'm an expert now. I could never reach my back teeth with normal flossing methods, and this solves that problem.
The criticism I read about the 450 are to me completely unfounded. My charge lasted for at least 5 days, and I used it twice every day. But it's no hassle charging it anyway, so why this is a criticism I do now know. Also, some have stated the water tank is too small. Well, I don't think my mouth is any smaller than anyone elses, and I find the amount of water it holds absolutely spot on. Some said it was 'flimsy', of poor quality. It is made of plastic of course, to make it lighter to hold I assume, and I am storing it in a safe place to ensure I don't knock it over by accident - but I can't see why that is a problem.
My only slight criticism is it would be good to have an indicator light to show when the unit is fully charged or near to the end of it's charge. This is a slight problem though, and wouldn't say this is a good enough reason not to buy.
In fact, I'm so impressed, I've purchased another for my son (birthday present), who has problems flossing as his teeth are quite close together. I know he will be delighted.
Please don't take on board the negative reviews. This product is excellent.