I played Rifts for years. I love the setting, but hated the rules. Basically humanity had advanced pretty far with mecha, cybernetics, and psionics, and then a massive nuclear was devastated the world. The resulting psychic backlash of life force energy released from millions of souls dying at the same time, super charged the mystic ley lines which run across the Earth.
And in many places the surge of psychic energy was so powerful that rifts to other dimensions opened and all sorts of beings came through. Beings from D&Desque fantasy worlds, alien races, demons, monsters, and assorted cthuluesque monsters, you name it. The world was plunged into a monster ravaged apocalypse for a couple hundred years. Eventually a few human and D-Bee (slang for dimensional being; aliens who came to Earth through rifts) communities have clawed themselves back into a semblance of civilization. Some communities have embraced magic, other technology. Some of these nations are fairly powerful and their mecha and weapons technology has exceeded even pre-rifts levels. The most famous of these is the Coalition States, a Nazi-esque xenophobic nation determined to reclaim Earth for humanity and purge all magic and D-Bees. There are magic based nations like the re-risen continent of Atlantis ruled by a cuthlueqsue alien entity (though somewhat similar to Jabba the Hutt) that runs what is effectively an interdimensional crime syndicate.
The cool thing about Rifts is there is a lot of style to the game. You can play cyborg soldiers, drug-enhanced super soldiers known as Juicers, mages, mecha pilots, or any number of super human mutant or alien races.
However, there is no balance to the system whatsoever. Some races and classes are vastly more powerful than others. All balance must be enforced by the GM in terms of allowing specific classes or races.
Rifts also emphasizes a concept called megadamage. Basically most of the supernatural nasties and most mecha and high tech body armor have megadamag capacity or MDC. Most supernatural nasties and high tech weapons like rail guns, plasma rifles, and so forth, deal megadamage. Basically 1 point of MD is equal to 100 regular hitpoints. And only weapons that deal megadamage can even harm structures, vehicles, or beings with natural MDC hitpoints. To put it in d20 Modern or D&D terms, your average 10th level guy may have 80 hit points. But in Rifts he could be wearing 100 MDC body armor which effectively makes him invulnerable to all mundane damage (100 x 100 = 10,000 hitpoints). However, he may be wielding a plasma rifle which does 1d6 x10 MD. So it would take him several shots to kill someone wearing similar armor. But if he shoots something mundane like an unarmored human than he does anywhere from 1000 to 6000 points of regular damage per shot. Enough to vaporize an unarmored human, or blow a car in half (assuming it has 500 hit points).
Basically in Rifts, if you can't dish or take MD then you better run. Combat (if you can get past the clunky rules) is very anime-like (think Dragonball or Fist of the Northstar) with buildings getting blown to rubble, and huge craters everywhere after a battle.
Magic on the whole is far weaker than technology when it comes to dishing damage, but alien beings, demons, and so forth, tend be tougher than most most mecha and MDC power armor.
Another thing to note is the power creep through some of the books. The power of magic and technology begins to rise through the first few books, until they began to reach a level of parity, but then Kevin Siembieda (owner of Palladium) took back control of the line and immediately dropped the power level back down again. A jarring change that never sat well with me. Its one of the reasons I stopped playing Rifts. The power creep climaxes in the two Phase World books. However, despite the power creep, the author most responsible for it, CJ Carella, did write the best Rifts books in my opinion. Phase World is still my favorite Rifts sourcebook of all time.
I like the source material but the system is in bad need of an overhaul. If Kevin Siembieda were to release a well-written d20 version, they would sell a bajillion copies. But I think that will never happen as long as Siembieda is in charge. He has an extreme irrational hatred of d20 (even though his system is a clunky rip-off of AD&D).
Basically, I think the system would work exceptionally well using d20 Modern/Future rules. All you have to do really is figure out how you want to handle MD and the rest should be fairly easy to convert. Just bear in mind that many playable races and creatures will have vastly different ECLs and some character classes will be far more powerful than others.