Anyway, that's more about d20 overall; d20 Modern in particular is a nice modular building kit that can emulate almost any kind of game you'd ever want to play. In fact, I remember thinking at one point that it could be the last game I ever play, since as a d20 fan, I don't need more systems and d20 Modern is so versatile. The campaign models (mini-setting cliff's notes, really) from the main book don't really highlight this diversity all that well, since they are all too similar to each other, but when you start looking at the Polyhedron Magazine d20 mini-games, most of which were built on a d20 Modern platform, and the campaign models that were built out of the d20 Future and d20 Apocalypse (and others) supplements, this versatility becomes more clear.
It was also fun to see d20 Modern adapted to other settings from other games that were reasonably well known and well loved. In some cases, this was done via copycat campaign models; i.e., the same themes and tone as a well known property but none (or little) of the intellectual property. These adaptations included Dark•Matter, Star*Drive, Star Law (which was Star Frontiers), Bughunters (which was Aliens/Starship Troopers), The Wasteland (Mad Max), Omega World (Gamma World), Hi Jinx (Scooby Doo), Shadow Stalkers (Dracula),Deathnet (Tron), Iron Lords of Jupiter (Barsoom) and even more. The ease with which d20 Modern can be adapted to almost any genre and almost any setting is only rivaled by GURPS; but it has the added advantage of also being compatible (mostly) with D&D, so D&D players can get up to speed on the rules after about five minutes of looking at the differences, and much of your D&D material can be used (if appropriate for the setting) including monsters, spells, magic items, etc. And although there are certainly pros and cons to each, I just kinda like d20 better than GURPS anyway. Although it's interesting to me that d20 Modern is positioned... kinda... as the GURPS of the d20 system.
There are, however, a few "holes" in what d20 Modern has done. I'm not aware of any significant efforts (and none by Wizards of the Coast) to use d20 Modern for a superheroes genre, for instance (although it looks like a d20 Modern sourcebook called d20 Spectaculars was on the product schedule at one point that would have addressed this.) And oddly, with the exception of the obviously starfaring science fiction settings, all d20 Modern settings assume the actual, real planet Earth as the setting; there aren't any fantasy settings. Wizards of the Coast can certainly be excused from wanting to pursue this, as it would conflict with their flagship Dungeons & Dragons games, but it always surprised me that I never saw any evidence that anyone other than me was considering taking d20 Modern + d20 Past and applying that to a home-made fantasy setting. It also surprised and somewhat disappointed me that the Modern SRD got very little attention from publishers and customers alike, too.
However, there seems to be a reasonably active fan community for d20 Modern at http://www.d20moderndb.com/ who have, among other things, come up with a Pathfinder Modern SRD, house rules, and more fan-created ad-ons. So, check 'em out. In any case, my decision to return to an earlier idea and use d20 Past as the rulebase for my "preferred" method of running Dark•Heritage (mostly because there are a lot fewer houserules and exceptions that way) reminds me again of how underappreciated this rule system always seemed to be.