USMC LAVs have held up to IEDs surprizingly well too, even though they dont have the slat cages. I have seen many LAVs that look destroyed, but the damage is really just the wheels all tore up. In most cases, the Marines inside were all OK, if rattled and suffering minor injuries. VBIEDs tend to be a little worse, since they explode right next to vehicle and not below it.
But better armor also means that the enemy will build bigger bombs. Awhile ago, 2/25 had a Amtrack that ran over an IED made of stacked AT mines. Shredded the vehicle and killed 25 I think. I remember one IED attack in Ramadi against a M113 that 1st Bde, 1st Inf Div had that was completely destroyed. All the soldiers were KIA and the biggest piece recovered was the ramp.
Its amazing that so much ordnance is still available in Iraq.