I never really got to play d20 Modern despite owning a number of the books, testing some builds and encounters out on my own, and plotting out some one-shots.
d20 Modern was an interesting line that show how you could pound a d20 system square peg into a round hole. I say that lovingly, because as the supplements showed you really could use d20 for nearly everything. It may have been a bit flawed, but liking something despite the flaws is a good way to be happy.
It also arrived during the d20 boom. So any accomplished d20 player could easily pick it up and run all sorts of games. For me, that was pretty exciting. I was excited by the prospect of running one-shots in modern, historical, or sci-fi settings and not have to teach a new ruleset. Granted it was far from the first generic RPG, but coming from a gaming group that only really played D&D 3/3.5, it made pitching something different possible.
The trickling of Star Drive and Dark Matter material in the d20 Modern books ended up leading to my rediscovery of Alternity and eventually purchasing the entire line via eBay after it was out of print for a few years. I ended up selling off my d20 Modern books, but held onto the Alternity ones.
Mark me down as another person that would love to see an Urban Arcana 5e. Of course, I'd love to see it updated from its early 2000s vibe (which is fun to check out again, just for kicks).
Of course, probably the best thing about d20 Modern is that it layed the groundwork for my favorite iteration of the d20 rules in Star Wars Saga edition.