I must admit, when it first arrived, the mouthpiece was sealed. After some research I discovered this is quite common. I mangled it in my fingers to no avail so then i nipped it with my teeth and it opened. The seam is quite clear so all you need to do is give it a bit of persuasion. The material is quite tough so don't be too afraid, although I wouldn't use a knife as you would risk splitting the rubber further than intended which would damage the mechanism irreparably and may lead to leaking.
Anyway, small hiccup over... I rinsed it out and used it straight away and haven't stopped! I drink much more water now and take this everywhere. I chose this size as it would keep me going for longer whereas the smaller size I'd say would just last as an accompaniment to a meal. I'd recommend this for work or if you're leaving the house for a day.
Colour is lovely - very true to the image. Tough plastic body. Doesn't leak at all and I've tested rigorously...