The general gist of this play among commentators on Goodreads is that much of the humour is dated which is why they don't think the play works all that well. It is not so much that people seem to hate the play, but rather feel that the content belongs to the past. That, and the fact that Johnson is overshadowed by Shakespeare, though I would suggest that Johnson wrote in the generation after Shakespeare, meaning that while he was a contemporary, it seems that his career is mostly post-Shakespeare, and Johnson probably wrote up to the time when the Puritans closed down all of the theatres in England. In any case it is always going to be difficult to write plays that last when you are up against a behemoth like Shakespeare, since Shakespeare was always going to attract most of the attention, and most of your work is going to be left to those who what to explore the literature that existed around his time (much like me). In a way it is sort of like competing with a reviewer like Manny on Goodreads, who has such a huge following that pretty much the rest of us pail in comparison.
What attracted me to this play though was that it seemed to deal with ordinary people. Okay, Shakespeare had ordinary people in his plays as well, such as the soldiers in Henry V and Falstaff and company in the two Henry IV plays. However, the main focus of his plays tended to be on the princes and kings (with maybe the exception of Henry IV where Prince Harry spends a lot of time mingling with the commoners). I remember one of the things that came out of history when I was in university was that there was the development of an interest in the lives of the commoners. In a way, history was moving away from being little more than dates and dead people, and beginning to be a sociological examination of the lives of people at the time. For instance I remember that in high school and university, the lecturer would give us a lecture on the common feudal village that existed at the time. However, with me, being young, impressionable, and into Dungeons and Dragons, when I did history all I wanted to learn about wars and heroes, not about how peasant kids would be beaten by their fathers in the paddock so that they would know where to grow their crops.
However, as I grew up I come to understand that there is more to history than simply heroes and wars, and that the voice and life of the common people are actually quite silent. This changes as more people became literate and the ability to write developed, however back in Shakespeare's time the little people, namely the illiterate ones, would simply remain hidden from our eyes, and when they appeared, they would appear as the laughing stock, the comic relief, and the foil in the plans of the hero. We see this in Henry IV and we see this here, where the actual lives of the commoners are hidden behind a farcial display of comedy and tomfoolery.
The other thing that I wish to raise with regards to this play is that question of making mockery of alchemy. The idea behind alchemy may be dated, but the idea of mocking it is not. Take for instance these guys:
Okay, this may not necessarily be what we have in the Alchemist (particularly since those guys are not a bunch of commoners that are trying to make money off of conning people into thinking that they can cure diseases and turn lead into gold) but it does show how playwrights and producers bring science, and even psuedo-science, into the world of comedy. In fact, much of what those guys (Sheldon et al) are studying could actually be considered psuedo-science since much of it is based upon mathematical assumptions and hypothesis, and in many cases these theories are little more than educated guesses. For instance, Einstein's theory of relativity is actually still just that, a theory.