This bottle has 2 closing mechanisms - a push button, and a backup metal clip. I once put my bottle in my bag, full, upside down by mistake, using the clip closing mechanism only, and only realised 5 hours later when I went to use it. It did not leak.
I've owned it for six months now, used it virtually every day, and not once has it leaked.
It also has no plastic taste. Until now I have avoided plastic bottles because I believed that all plastic bottles make the contents taste of plastic.
One advantage of the narrow-neck OTF over a wide neck Nalgene bottle - I once knocked the OTF over. I was surprised how little of the contents I spilt - it simply can't pour out very quickly because the neck isn't very large (approximately 2.5 cm in diameter).
May I suggest that you don't reuse plastic supermarket water bottles for your kids' packed lunches (or your own)? It's not recommended, apparently, because of the type of plastic used for the bottle. Supermarket bottles are, apparently, not intended for anything more than one use. I have no idea if the health scares are true - I just believe that an expensive bottle is more likely to be safer than a 50p one.
Incidentally, I wouldn't use the OTF for hillwalking as they are not unbreakable. If you drop it accidentally and it breaks, you've got a major problem. Personally I use an aluminium bottle for hillwalking. But for everyday drinking I use the OTF as it is so much better designed than an aluminium bottle.
My only gripe - the base is slightly too broad to fit in my car cup holder. In case you want to know if it'll fit in yours (and being far too anal here for my own good), I've just measured it - it is approximately 7.2 cm in diameter.
I really like this bottle.