The "heter" to shave using and electric shaver is completely dependent on the interpretation of our gemara. Much has been written to prove that shaving is prohibited, and R. Moshe has always been the person to blame the heter on. Many of the distinctions that are made in determining which shavers are permissible such as the closeness of the shave, lift and cut, rotary vs. screen - are all not so relevant. Those who forbid electric shavers would forbid all types, and those who permit should permit all types.
The gemara explains that there are 2 contradictory verbs used in conjunction with this prohibition: לא תשחית - Don't destroy (the hair), and לא יגלחו - Don't shave. The gemara explains that if we were to forbid all "destruction", then tweezers would also be forbidden. If we were to forbid all "shaving", then even scissors would be assur. However, the contradiction in terms yields an outcome that the only thing which is forbidden is גילוח שיש בו השחתה - when there is a combination of both shaving and destruction, which can only be achieved with a razor (הוי אומר זה תער). The Rivan (rashi) explains that it is only forbidden if it is normal to shave in that method and accomplishes the destruction of the hair by cutting it close to the root - tweezers destroy but are not normal to shave with, and scissors are normal but don't destroy.
The introduction of an electric shaver that shaves very close would likely qualify as "destruction". Although a razor may be slightly closer than the best of electric shavers, it is difficult to accept that the minor difference would be sufficient to no longer consider this a "destruction" of the hair. Therefore, the many poskim who considered electric shavers to be problematic (i.e. chofetz chaim in likutei halachos) is because they achieve the goal of both "destruction" and qualify as "shaving". The gemara didn't recognize anything that could achieve "destruction" other than a razor, but since the Torah doesn't use the term "razor" - we would be forced to include the electric shaver in the category of גילוח שיש בו השחתה which is assur d'oraysa.
I consulted R. Nota Greenblatt on this issue. He said that although R. Moshe never wrote a teshuva about this, he was adamant about it being mutar. "R. Moshe gave a haskama to every ne'bech that asked, but would under no circumstances give a haskama to the sefer hadras panim which prohibits electric shavers". He explained that R. Moshe didn't care about how much hair is left at the end, because he held that a תער - razor, is assur, anything else is mutar. The difficulty with this approach is that it doesn't say "razor" in the Torah? It seems that R. Moshe held that the contradiction between the pesukim led the gemara to conclude the type of action that is prohibited, not the result. It may be true that the result of an electric shaver is identical to that of a razor, but the mechanics used in the hair removal is completely different. The razor rubs against the skin and cuts the hair with one blade, whereas the scissors doesn't rub against the skin rather utilizes the rubbing of 2 blades together to cut. The electric razor which rubs close to the screen and therefore successfully cuts the hair, simulates the scissor action, not the single blade action. This scissor motion is categorized in the language of the shulchan aruch (181:9) as מספרים כעין תער (scissor motion but close as a razor) and is permitted. It should follow that by following R. Moshe's approach, there should be no reason to test the sharpness of the blade, or remove the "lifter", or discuss the closeness of the shave. Being that the mechanics of an electric shaver is a scissor action, they should all be permitted.
Perhaps those who test the shavers are concerned that if the blade is sharp enough to cut by itself, it may be doing just that. This approach is very difficult to understand since the blade is not rubbing against the skin, and not even rubbing against the screen (if it rubbed against the screen, it would sound like metal scraping against metal). Furthermore, the methods that are used to test the shavers don't seem to be valid forms of testing whether the blade could cut the much thicker and stiffer facial hair.