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Backpacker Magazine tested 5 Beer Growlers. And the news is good.
The Contenders
Drinktanks 64OZ Growler
Kegworks Double-Walled SS Growler
Innate Craft Growler
MiiR 64 oz Vacuum Insulated Growler
Hydro Flask 64 oz Beer Growler.
Stanley will be introducing a Growler later this year but it was not available for testing.

After thorough scientific testing and field testing and taste tasting as described in the article, the authors concluded:

The Results
Sorry, internet: no disastrous fails here. “I have seen many good beers become oxidized and flat, even in just a 24 hour period,” said Brew Master Scruby, “but the growlers tested did a great job preserving the flavor of the beer and retaining the carbonation.” All the stainless steel insulated beer growlers we tested gave us very drinkable brew after 24 hours (including time in the sun and being bounced around in a pack). We saw no leakage from any of the growlers, but, with the help of our master brewer, we were able to tease out the ways in which each one was subtly different. And we went ahead and ranked them, because what good is a test without rankings?

They ranked Most Affordable, Lightest, Best Value and settled on their Beer Snob's pick - The MiiR 64 oz Vacuum Insulated Growler.

Shameless plug, you can now take that wonderful Arizona Wilderness Brewing Craft beer with you on trips.

http://www.backpacker....

Growlers and pistachios are a great combo.

Not that I have shopped much to know all the nuances, but I use (and have given many as gifts) the Fifty-Fifty growler. ( dead link removed )
On a website named 'theclymb.com' they sometimes have them @ about $25. I can vouch that they DO hold the carbonation.

These appear to be well-priced - shipping decreases proportionally with each additional growler. ( dead link removed )

i like to use a 1 or 2-liter soda bottle, and a carbonator cap. from my taps at home i can turn the serving pressure down to 2psi and fill the bottle, squeeze out the air, place the cap and then burst with 15lbs co2 from my tank. it's the lightest growler around.. not insulated but i envelope it in goose down ;) and that protects the beverage from UV light (skunking) too.
google "carbonator cap" - amazon, $12

Tempe Costco has Reduce double walled SS growlers for $30. I got a couple 2 weeks ago and so far they're pretty solid.

wait a minute.. is it even legal to fill a growler that isnt made of glass? i dont recall hearing that stainless or metal of any kind was legal for a brewhouse, corner store, or otherwise to even fill up.. but it's been a long day. someone correct me!

@te-wa
that was fixed one year after the original growler law went into effect.

Also, the ones at Costco have the tops that are detachable. They say it's for if you have coffee in it, but it makes cleaning it much easier

Ok ok some how much time do you have to finish off the Growler before it goes flat?

@Widowmaker
Don't worry about it. For you it will never be an issue.