I realise I seem to be in a minority (of one?!) on this topic, but for the sake of balance can I just add my experience of Trimetals bike garages. I've had one for about two years now; its not flimsy, its not cheap and Trimetals were a helpful company to deal with.
Occasionally I get a little condensation, but it really is very little cos the garage is well ventilated. It would have been far cheaper to buy a timber shed, but my main considerations were security and fire risk (in my line of work I've seen too many timber sheds go up in flames!). My metal bike garage covers both issues admirably and far outweighs a tiny drop of condensation.
As with the aquisition of anything, it depends on your own circumstances and/or priorities