i have 2 greenhouses now and will have a third by this winter most likely. i will say a few things.
i am not a fan of those pre made greenhouses, even the more expensive ones. every greenhouse should be site and use specific. that is it should be designed to fit the location it will be in, the environment, and what you will use it for.
another issue with those greenhouses is the heating and cooling. chances are it will be too hot when its hot, and still cold when its cold. oh and you have to water even when it rains.
judging by your first post if you are going to be starting seeds inside and bringing them out. a cold frame or even a pvc hoop tunnel will provide you with better results for cheaper.
if you want to overwinter plants or start seeds outside early, I start mine in early January. even before some people start theirs indoors under lights. for this the greenhouse has to be placed properly and extra measures taken by building a germination chamber aka sweat chamber( there is a thread here on it)
if you like buying stuff and enjoy showing it off to friends and such, those greenhouses are great for that.
For all things come from earth, and all things end by becoming earth.