I bought the four-tier greenhouse as a way of propogating seeds, and to keep half-hardy plants from frost damage over the winter. It is very easy to construct, and does it's job, in keeping warmth and moisture in very well. The plastic cover fits right to the bottom of the frame, which means it is very snug for the plants in there. Ulike other reviewers of this type of greemhouse, I have not found a problem with the zips coming apart from the cover - perhaps the manufacturers have corrected this fault. The only problem is that the frame,constructed from metal tubing and plastic, is a bit wobbly, as the joints can't be tightened up in any way. I had three planter trays on each shelf, and the whole structure veered rather alarmingly to the left, so I had to attach it with string to the wall to prevent it falling over. I also put some plastic sheet on each shelf, because I found that when watering, the drips disturbed the soil in the seed trays below.
Still, what do you expect for £20? It more or less does its job, and my lettuces are now coming on well.