Review (4/5): two is just a starting point
they're OK - the main thing is that you really can't do much with just two at a time - if you use one on each side, you end up with more of one little hill on each side. with two on one side you can get a bit more of a cascade volume fall, but then you have to stop and do them on the other side. at a minimum, two sets will get you some decent volume up front or whatever section you choose to use them - i think though i would probably prefer 6-8 to create a total up top volume look - 2-3 more sets of these. the next kind of issue is remembering to use them - i'm straightening my hair if i am drying it and you really have these in during that process because of the smoothing brush process that's part of that - so then i have to remember once i am done drying to go back and use these before i do any flat iron or curling things.
to get some up top volume - they do kind of work - i would just suggest determining how much volume you prefer in terms of where all on the crown and order based on that - for me it's 2 on each side up front, then 2 more rows of one behind it.