I first heard of Klean Kanteen via Lovely Bicycle. They are available here in the UK (including in a few bricks and mortar shops) but as they are made for the US market, their capacities are made to crazy US units, rather than sensible, rest-of-the-world metric.
Vacuum flasks are nothing new, but the majority of vacuum flasks on the market are aimed at people who want to carry hot, still drinks and are often sold with lids intended to double up as cups to drink from. Something I wanted for a long time is a vacuum flask for cold, possibly fizzy drinks and which I could drink from directly whilst on the go, rather than using a separate cup. One of my main aims was to reduce the number of cold drinks I purchased whilst out and about on warmer days, partly to save money and partly because of the tendency of many shops to use broken/ineffective drinks chillers (Boots and Superdrug, I am looking at you).
Eventually, I found out that Klean Kanteen made insulated versions of some of their bottle sizes. The size I bought (nominally 591 ml, 600 ml in practice) came with a loop carabiner lid, and I was not sure if the lid would be ok with fizzy drinks. However, I saw that a larger version of the same bottle (but with a swing-top lid) was being sold by Klean Kanteen as a growler* for carrying (fizzy) beer.
For a while I used the loop carabiner lid, but the seal on this did not do very well with carbonated drinks; the lid had to be screwed down very tight to prevent the pressure leaking out and when it was opened, a small part of the seal would often unseat before the rest, sometimes resulting in a highly directional spray (especially after riding the Brompton over cobbles). This happened in quite spectacular fashion as a sat in the quiet coach of a Virgin Pendolino to Edinburgh, spraying two fellow passengers with a fine mist of Pepsi Max. After this, I decided to give the Swing-top lid sold with the Klean Kanteen growlers a try instead. The addition of the swing-lock cap produces an ideal solution for keeping cold, fizzy drinks cold and fizzy for an extended period of time (6+ hours has been no problem). Occasionally, I have also had coffee in the bottle, which stays hot enough for me for at least four hours (and possibly a fair bit more too). A few times, I have even used it for beer.
Another feature of the flask which I like is that it is constructed from 18/8 stainless steel, inside and out. Whilst I suspect that the recent hysteria about BPA is overblown, I do not like the flavour-retention which happens with re-usable plastic bottles. Aluminium bottles can also have this problem, as they require lining and this lining is usually made of some sort of plastic. The advantage of stainless steel is that it does not retain or impart any flavour on the contents of the bottle and is resistant to corrosion from the sorts of things you are likely to store inside the bottle and also the sorts of conditions the outside is likely to be exposed to. It is also durable, I have dropped mine a few times and whilst it is dented, it still functions perfectly well.
This bottle really works well for me, I use it everyday. I even took it with me when I had to travel to Japan for work in early May, which allowed me to really maximise the benefit from the airport lounge access I got as a random bonus checking in.
*Growlers are vessels for transporting draught beer from a bar or brewery for later consumption elsewhere. I suspect that the name may be hindering their uptake in the UK somewhat.