Any actual play experience with either/both of these? I love the pulpy goodness and Slavelords of Cydonia is on clearance at the FLGS.

Moreover, how do these systems compare to D&D, Iron Heroes, Conan, etc? Would you run a high-fantasy/lower-magic game with them? To be honest, I'm getting kinda tired with the magic-item-dependance of most classes in 3.5...

Well. I would definately run a lower-magic game with it. And if you toggled off some of the more dangerous options--hit point limitations and stuff--you could keep a high-fantasy feel.

GT is built to be a very modular add-on to Mo-Dern. They have a handy lethality scale attached to different options. I like to play GT with the danger dialed all the way up. But there's nothing to say you can't just add in the low, dangerous magic options, and leave the other settings on low heat.

I recently got Grim Tales at a clearance sale and I have to say that it's quite nice, but nowadays I'd probably go with other options. True20 comes to mind if you do want generic characters without the hassle. Iron Heroes for more cinematic combat. Or lately, just straight D&D. With the Tome of Battle classes, who needs magic items? You could have a very interesting campaign just with that book, especially if you expand on the disciplines a little. I'm currently waiting for the marble to finish my Mike Mearls altar.

Edit: And I just have to mention that GT has a quite a bad layout. It looks like it was designed using Word...

Late coming to this but I hate seeing anyone recommend True20 over GT and not say something...

D20 Modern has some great support, you can do a lot with it. Alas, this support is scattered about many books. But if you can track down just what you need for what you are imagining, it's a pretty sweet set up. I particularly recommend Sidewinder:Recoiled, Darwin's World 2, and Blood & Fists.

Grim Tales is basically a retooling of D20 Modern. Some of its conventions I don't like (like eschewing advanced/prestige classes), but its very flexible, and highly suitable for dark fantasy or horror. The flexibility comes in that at several junctures, variant rules are provided with different "skull ratings". You pick the way you want it to run according to how grim and gritty you want it to be.

I feel that both of these are superior games to the much baleyhooed True20. True20 is actually more complicated in play than either, doesn't have near as good setting and system support than D20 Modern, and lacks the great feel of the GT horror rules and "skull rating" rules.