I have a couple of these racks at home. They're heavy, constructed out of mild (bendy) steel, but appear to hold up OK if treated right. As the name on the website says, they are actually free-standing, which means that when you put a bike on the top hanger, the whole rack leans forward at you until it reaches the new balance point. A little disconcerting, but not a big problem when you're used to it.
Strangely angled top tubes shouldn't be a problem, since you can independently adjust the level of each of the arms. I did find that the racks are only just big enough to hold my bikes, which are all large. I get a bit of annoying interference between the wheels of the bike on top and the saddle of the bike below. But liveable at such a cheap price.
One other thing - I chopped up an old MTB tube and slid sections onto the bare steel hands that hold the top tube to reduce paint scratches.
I think that the best storage method, in that it is most space-saving and potentially very cheap, is the hook in the wall that Weiyun has shown. All you need is a wall that you can do what you like to. All the European trains (with proper bicycle compartments) use this method (including the special
1950s English bicycle carriages), since it's the most economical.
