The Ikea ones are fine, indeed the best I have found. They have a soft cotton like side, and a more slippery side. DH puts them on the wrong way frequently, but does not affect performance and the kids do not complain. They are not plasticky and not overly sweaty. I would use them all the time straight from the beginning unless you have a fancy breathable mattress set-up. They save having to wash the mattress cover, if you have one. If you don't, then they save ruining the mattress. I have been using mine for about 5 years, and even put them in dryer, and they are still fine. Hippy Chick do some but at £20 for the same size, I would not get them, as you need a few. They do fitted ones that you may prefer - the Ikea one is flat.
The advice is to keep baby in with you for first 6 months to reduce risk of cot death. Apparently the C02 in the room from you helps them to remember to breathe. Mine were in their own room very early on, and I got a movement monitor as a safeguard. Unless you go to bed when they do, and stay when they nap, the safeguard is not there, and that would drive me crazy!