Omlet coops are unbelievably overpriced, undersized, and underventilated. Not a good choice.
I have one. It works well. It's worked well for the last 2 years. My chickens even choose it over the wood built coop. Its basically a secondary coop since I can't build myself, but I wanted more chickens due to the huge run we had built by contractors we had plenty of space. Hubby said he wasn't building me another coop (he's no builder so the first one was plenty for him). I was able to put the eglu together myself. The chickens have faired well through summer and winter. We get extreme temps and humidity on both ends.
I got it for 4 chickens, but 6 decide to roost in there. I have no issues with ventilation. I had one hen get a mm section of frostbite on one of the points of her comb (I didn't measure, but is a pretty tiny spot). I've had some chickens get frostbite in my wood built coop too - my wood built coop has plenty of ventilation (built to byc standards). I think with the omlet, people only see the vents (there are 3 in the eglu), but what most people don't realize that all around the perimeter under the top overhang are slots that also provide ventilation.
Now, you would not be able to let chickens just hang out in the eglu if your intention is to lock them in it. But, my run provides all the security, the windbreaks, and the hangouts a chicken (or duck) could want. They use the eglu to roost and to lay eggs. I haven't modified anything.
In the summer I add a fan, but I'm not sure it does much good. I just add it because I read that so many think the omlet coops aren't good enough. But, I have put a thermometer inside and in my situation (with adequate shade) the temps inside have been a bit cooler than the ambient temps in the summer. I've not measured it in the winter though.
It's definitely not everyone's choice. It is expensive, I will give you that. But, I haven't found anything in the chicken world that isn't. Once you think you've got it all figured out, something needs fixed or a chicken gets sick.