I will always love 'The Eugene Trilogy', Lost in Yonkers, and Barefoot in the Park. I recognize it is an American classic, but I never much liked The Odd Couple (and I saw it with Art Carney and Walter Matthau, among other casts). I just didn't think lines like 'its either new cheese or old meat' (close enough) were funny. So, just based on those works, his place in theatre history would be well assured and deserved.
That said, I am amazed how many of his hit plays are dated to the point of being unwatchable. I recently watched California Suite and Plaza Suite on TCM and -- although I stuck to the end -- they were both pretty much unwatchable. The humor is lame, the situations forced, etc. With the exception of Maggie Smith, Jane Fonda, Barbara Harris and Michael Caine, all of the acting was over the top in a bad way...and Fonda and Harris's vignettes themselves were positively awful.
Same thing IMO with Prisoner of Second Avenue and Last of the Red Hot Lovers, both big hits, when I saw them in Summer Stock (in solid productions, with Broadway veterans in key roles).
I imagine that he will always be respected for the first plays I mentioned above, for his ability to turn out hits in the period that was in retrospect the beginning of the end for comedy on Broadway, etc., but that he will be viewed as the successor to Philip Barry, most of whose plays are also terribly dated although Philadelphia Story and Holiday will always be classics.