Yes, I saw those reviews, too! I have to chuckle. The first waterer I ever used was the same style as these without the heated part. I vividly remember trying to fill the thing without getting soaked myself! After a few times of giving myself a bath with apple cider vinegar water, I figured out you have to be smarter than the plastic. lol
I now carry two gallon pitchers out to the pen with a small funnel. I turn the waterer upside down, and yes, a little of the water pours out. I use that to my advantage, because as we all know, chickens dirty their water, so this lets me rinse the basin and clean it a bit right there. Using a funnel makes for a much drier filling up. The stoppers are very secure, even in the frigid cold. They haven't leaked for me. When I turn the waterer back up, I figured out if I turn the side with the little water channel side going over the top and down (if that makes sense), then less water comes out.
Now, I also learned the hard way (as did my 11 year old son) that you probably shouldn't carry three gallons of water by hanging on to the base. It is inevitable that the weight and the movement of you walking is going to be a fun disaster and the base is going to give. I don't carry it that way anymore. The days that we bring the whole thing in to wash it and fill it inside, we carry it by holding the whole thing. No more fun disasters.
And yes, if it is unlevel, water will pour out, at different speeds depending on how unlevel it is. This is true even for pans. That is why they come with a handy hanging rod. Hang it up, gravity levels it, no leaking.
So in short, yes, the first time I used a waterer of this style, there was a time of learning the best way to handle them. But once I figured it out, operator errors became fewer and more far between. The only reason I like this particular waterer over the one I bought last year is this one is a bit easier to secure the base back on after washing and filling. The first one you really had to work to line up the tabs. Not fun when it is super cold outside, and the stopper (for funnel usage) was a pain to secure, while this new one is much easier.
Last year, we were below zero every day for two-three weeks straight, and then several times after that. The only time the water froze was when there was just a little bit of water in there. I don't let them get that low anymore. This year, we have gotten below zero, so I can't say how the new model will do yet. But in the single digits, it has done beautifully.
So I guess all that is to say the majority of those negative reviews are truly due to operator error and people like me learning how to best work the things. I hate that people give bad reviews based on operator errors, instead of just laugh at themselves and go dry off like my son and I do.