Well... I've used the Grim n' Gritty rules now for a while, and I have few complaints.
It adresses my main concerns with 3.x, mainly how armor works, and how abstract the system becomes. My players definatelly like GnG, as it reminds them of an old system we used to play.
The only real problem, is when you have either characters in the heavier armor, which provides around 7-8 protection, meaning that the archer players are pretty much *fskd.* In my trial, where we checked if the system worked for us, we player a bit of Ravenloft. The only problem there was that most of the creatures had high natural armor scores, and even DR to back it up. I put them originally against a chain devil, which had 8 in natural armor, and 5 in DR/silver, meaning he "soaked" 13 from each hit... AND he had regeneration of 2... For 7 turn, they were doing nothing, but being toyed with. Half of the group ran away in fear when they saw the devil coming for them, so left was the TWF Ranger, the sword/shield knight and the spearwielding barbarian (as weird as that sounds). They started though playing smartly, and although they were almost dying, they tried fx. tripping him, flanking him, taking him on one-on-one. It ended when the sword/shield guy confirmed a double critical hit, whilst piercing natural armor. He did roughly 20 in damage to him, and then the TWF ranger stepped in, dealing 3 in damage.
So basically, watch out for heavily armored/natural armored creatures, as they do seriously wreck havoc!
Ohh yeah, we also made it so that you aren't restricted to just one called shot, as the iterative attacks usually don't hit when you're laying down a -8 penalty, and suffering -2 from light wounds. That, along with being lvl 6 and only having a +12 modifier to your primary attack. At higher levels, this could be a problem, but for me, it isn't.