I played it (several times) and liked it. It does suffer from the problems of 3era multiclassing (e.g., saves, attack bonus, etc.) that a 5e universal proficiency would do wonders for rectifying. I remember playing a gunslinger-psychic sniper in one game, and an investigator in another. I never got around to running one of my favorite mini-settings from Polyhedron: Iron Lords of Jupiter, which was a Barsoom/Flash Gordon-inspired planetary romance setting. I also liked a lot of the spin-offs that it inspired (e.g., Grim Tales, True20, Spycraft, etc.).
It feels clunky now, and I would probably look to other games first (e.g., Fate, Urban Modern Fantasy, Modern AGE, Cypher System, etc.), but I would still regard d20 Modern as one of the most important and influential books to come out of the d20 3Era.