Our LG rep came into the store in Madison today and had a G Flex on him that he let me check out. It feels every bit as big as what I thought it would be due to the 6" screen but it did feel really zippy and fast as expected.
To me it feels like a curved G2 with a big display which is good and bad. I really don't need a phone that big but some people do, so I get it.
Personally I wouldn't use one but for someone who doesn't want a Samsung phablet I would recommend them to try this one out. We'll see longevity wise how it holds up to being dropped and what not.