I'm much more a fan of Palladium Fantasy than Rifts. When Rifts first came out I thought, "Holy @$%!!! What did they do to Beyond the Supernatural?!?!"
On it's own though, it's a setting that's got all kinds of fun possibilities.
The mechanics? Yeah, there's some issues there. These days (for better or worse) people approach games with the inherent expectation/belief that it's all "balanced". Rifts expects that any kind of balance issues is going to be handled by the GM.
I've only got the main books and a few others. Picked up as lots. I know the Vampire Kingdoms is one of 'em. Atlantis is another. The books do seem prone to power-creep, but even D&D with it's fans all obsessed with being "balanced" has had that.
d20 is a logical enough system to try and use to run Rifts. The biggest problem I've seen with any Rifts conversion (and a lot of other conversions in general) is the attempt to convert the mechanics into a different system. Since the classes weren't really made with the idea of being balanced relative to each other, you wind up with an awful mess when you try and do the same thing in something like d20.
Of course, your flavor of d20 is going to affect how things look too.
I personally would start with either Grim Tales or BESMd20. Depends on the overall structure/flavor you want. After that, I'd add in different books depending on what I expected the game to feature a lot of.
Glitterboys/Coalition stuff, I'd go with Dream Pod 9's Mecha Compendium Deluxe. It's got a complete and consistent system to build all the tanks/glitterboys/etc. You could always add in the Guardians of Order d20 Mecha Military Vehicles book after that, if you want the modern style aircraft/tanks/etc. Since it's using the same base rules as the Mecha Compendium Deluxe, you should really have too many issues mixing 'em up.
Magic.... yeesh. Pick your favorite magic systems and plop 'em in. Everyone's got their own pet version of the "best" magic books out there. I personally would start with Unusual Core Classes: Spellweaver from Misfit Studios. That's going to be the skill based magic stuff. They convert the SRD spells over for you and overall it seems like a tight enough system.
Psionics, I'd go with Green Ronin's system which is found in the Psychic's Handbook, as well as one of their Advanced Guides. Again, it's skill based.
Point based magic stuff, you've got a few options. I personally would be starting with BESMd20 as a core, so I'd just stick with their approach to spells. Honestly, magic systems are one of the things that everyone and their dog has written a sourcebook of some sort for. It's all mainly a matter of personal preference and knowledge.
Stepping outside of d20?
Well, my "go to" system these days is Zorcerer of Zo. It's a pretty light system. In the case of Rifts it's advantage is that you don't really have to worry about "balance" in the same way. You just make "Glitterboy" be one of your stats, and agree with the GM about what exactly a Glitterboy is in the first place. Nice and simple. ZoZ is powered by the PDQ system, which also powers Truth & Justice and several other games. It's one of the few games where you really can have Batman and Superman hanging out and contributing just as much to the game. Not for everyone though.
Talislanta/Waste World. Yeah, it might sound odd but the old Waste World game has a number of similiarites to Rifts actually. The overall system seems to be pretty close to the system that's in Talislanta as well. Waste World though has a point buy thingy as part of it, so it'd be easier to build certain aspects of Rifts. I'd looke to Waste World to cover the techy stuff, and Talislanta to handle the magic stuff. The rules seem to be similar enough that you could decide which of them was going to be the "base" system, and then you'd import the other bits you wanted from the other system.
For a non-d20 approach, and something that's more conventional than PDQ or Talislanta, I'd look at Unisystem. I personally prefer the Cinematic version (Buffy/Angel) but you could go with the Classic (Witchcraft, All Flesh Must Be Eaten) too.
Use a combination of Witchcraft and The Magic Box ( you can still buy it easily as a pdf) to cover the magic side of things. I'd use Angel as the base set of rules, and bring across Armageddon to handle funky powers and the really big stuff. There's All Tomorrow's Zombies which will also help take care of creating other races, as well as cybernetics, other psionic rules, and all kinds of other sci-fi stuff. Armageddon: Armed Force is probably another book to consider. It covers the military side of things in the Armageddon world. There's bits that can be lifted from other Unisystem books, depending on exactly what you want to import.
Of course, you can always go with whatever your favorite supers game is.
The one thing that always seems to trip people up in looking at running Rifts using another system is the whole MegaDamage thing. I personally think it's a non-issue. Just drop it is the easiest solution. If you want to keep it for whatever reason, then a supers game is probably not a bad way to go. Most game systems don't really deal with damage on different scales. Meaning a knife, a gun, a car, and a tank will all do different amounts of damage, but it's all effectively at the same level. Supers games have to deal with the idea of super-damage vs "regular" damage. So just rewrite the classes to fit in the supers system. Glitterboys wind up being just another version of an Iron Man powersuit, the various casters simply do magic in different ways, some classes become the Scientist/Gadgeteer type, and so forth.
Just like people say, "I play D&D" and you have to wonder "What kind of D&D?", when someone says, "I love Rifts" you have to kinda wonder exactly what kind of Rifts game they're talking about. Like D&D, Rifts has so many different things going on with it, that you can't really expect everyone's on the same page.
Oh and in answer to the question "What is Rifts like?"...
It's like climing into a mecha and shooting it out with a bunch of other mecha.
It's like playing a character from some other dimension, and walking into town and having a bunch of people try to string you up.
It's like scrounging around in the wastelands, trying desperately to find something to eat.
It's like having a character that looks at Dr Strange and mutters "chump" before showing him how to get it done.
It's like playing a cyberenhanced character from Cyberpunk or Shadowrun that's lost all of its humanity, and had its power turned up to 11. Cybernetics optional.
It's like playing a normal person that's trying to hangout with a bunch of supers that are fighting each other, and trying not to get blown up in the process.
It's like fighting to liberate humanity from the oppressive grip of a cybered militant dictatorship.
It's like fighting to save humanity from the flood of strange creatures from other dimesnions looking to enslave humanity.
It's a total blast if you're willing to either put it into a system you like, or you don't worry about the system it comes with.
If you like John Woo, Die Hard, Aliens, or anything like it, you should seriously look at it and let your mind wander over the possibilities.
If you can't picture yourself saying, "Woohoo!" you'll probably want to give it a pass. If you just don't get the idea of a mech that's completely chromed and fires a gun massive enough to require stabilizers to shoot out from the feet of the mech and into the ground, the possibilities are probably going to shoot right by you.
