I've got that Hemnes daybed, it's great. We had a different ikea sofa bed which I hated and I wanted to get the daybed. Turned out our neighbour had the Hemnes daybed and were about to buy our exact same sofa bed, so we just swapped! They bought it from Jo Guest (90's glamour model, if anyone remembers!) so it's officially our (Jo) Guest bed. It's very deep to use as a sofa but I've amassed about 15 large cushions to go on it so it actually makes a very comfy nest!
LittlePandaBear - when it's closed it's basically a single bed with 3 sides and you have two mattresses stacked on it. The bit with the three drawers pulls out (just slides forwards) so that doubles the size and then you put one of the mattresses on those slats. Did that make any sense?! It isn't too heavy to pull out into a double bed unless you have stuffed the drawers as much as I have, but you need to dismantle it to move it from room to room and moving it around the room isn't something to plan to do regularly! (all the slats go out of alignment on our anyway)
Also, with Hemnes stuff, a quick update you can do is change the knobs. I personally like the grey ones but I've seen them with some really cute ones which makes it look more personalised/less mass-produced.
That's a mighty long post all about a day bed.