First, it is hard to get a controlled condition under which to test these things. Whether one feels any discomfort has a lot to do with the departure/landing flight path chosen by the pilot. Relatively faster ascents/descents give one's eustachian tubes less time to adjust to changes in pressure, especially if one has 'sticky' tubes that don't flex easily. There's also at least some variation in cabin pressurization from aircraft to aircraft. Add to this differences in relative health over time even within the same individual, and the number of variables becomes impossible to control. So plenty of people might in good faith tell you that they work, but the actual case might be that the type of condition that causes the most discomfort didn't exist at the time(s) that they used them.

To the extent that they 'work', my GP and I (I'm a pharmacologist by training) decided that these devices if inserted as prescribed trap a small amount of air in between the plug and the eardrum. This air pocket acts as a buffer between the cabin air, with its changing pressure (even though 'pressurized') and the ear drum. No plug can or should be a perfect seal between the cabin air and one's eardrum. Thus, what could be the case is that as cabin air and the air pocket slowly intermix, the pressure of this air pocket changes, but more slowly than otherwise, giving one's tubes (which are the other side of the ear drum in the middle, not the outer ear) more time to adjust. That little filter plug in the 'channel' of the Earplanes seems to be part of the means whereby the cabin air and the air pocket slowly intermix. This effect can be approximated using cheaper devices - but more about that in a minute.

This slower intermixing of air will NOT help if one has a cold - nothing helps in that situation except to take decongestants (and use moisturizing nasal spray and drink lots of water). This is because the only thing that ultimately relieves pressure discomfort in the long run is functioning Eustachian tubes. Thus, even the manufacturers of Earplanes specifically rule out colds as an appropriate situation to use these devices, since one's tubes are swollen when one has a cold (sometimes to the point of almost completely blocking one's ears). Best yet is not to fly at all, though of course that might not be an option.

You can approximate the effect of Earplanes for a lot less cost by inserting ordinary ear plugs *before* takeoff, thus trapping a pocket of air at ground level; once you reach cruising altitude you can loosen the fit of the plugs gradually, thus allowing increasing intermixing of air. Ultimately remove the plugs. Then, just before the descent starts, insert them once more, and repeat the procedure.