Relatively fast. Relatively reliable. Relatively protective. Lots of relatives there, but everythings a trade off. Moving the fuel tanks outside the main compartment was a good thing. Spall liners and bolting on armor for RPG's will help. Decent small arms protection
They're certainly more protective then a humvee and not that much more fragile then a Bradley or LAV when it comes to big IED's.
You want a really protective APC, you do what the Israeli's did and convert tank chassis (they also started the stand off armour to predetenate RPGs on their M113 way back when). Of course that gets expensive, they weigh a lot, and they're big. Everythings a trade off.
The big thing is that they are ready "RIGHT NOW" and can be up armoured and sent to the front cheaper and faster then anything else going. Ironicly, some M113 upgrade programs were cancelled in 2001. I wonder if what they are doing now was part of that cancellation.
The M113 wasn't the best answer in the world, but it does some things pretty well. Funny how so many tools of the '50's and early 1960's are still floating around pulling their weight.
Big enough IED and it won't matter what you're in anyway.