We had a polytunnel from First Tunnels installed about two years ago. It's heavy clay soil here and by the time the diggers had been on the plot to dig the new pond and level the area for the polytunnel, the soil was very compacted.
We dug two large trenches, built up the sides with some planks - and so have two raised/sunken beds which we filled in with imported top soil.
I grow tomatoes, cucumber, aubergines and peppers in there as well as bringing on all the seedlings in the spring. I also have a couple of nasturtium plants in the corner and marigold and basil companion plants.
This year I experimented with doing seedlings in gutter piping suspended to elbow height from the crop bars which was great. I will do much more seedlings like that next year as well as more hanging baskets of chillies and cherry toms.
We have sliding doors at either end and the lower half of one side has an adjustable vent.
I water into upturned two litre pop bottles which are sunk into the soil near the plant roots which keeps the bed surface dry and makes the weeds very easy to pull out. Plant feeding consists of just chucking in half a teaspoon of feed granules in the bottom of the pop bottles and then watering it in with the hose. Seems to work very well.
Each October I turn the previous year's compost onto the beds where it sits in the dry over the winter. I use some of it but most gets raked over in the spring and taken down into the beds.
I don't do much in it over the winter except to keep the current year's strawberry daughters, the herb pots and the wormery dry in there.
I am making plans to get another one soon. Like FW says - one you have one you immediately realise that you need another one.
I love my polytunnel and have a little stool so that I can just sit in there on rainy summer days and inhale the scent of the tomatoes.