Not having experience with every system and lens ever made, I've looked into this a while back and the real advantage is only in the super telephoto lens range.. I think beyond about 300mm or so.

The test is(should be?? ) quite easy to perform. Take a lens like the Sigma 150-500 which comes in an OS version for Canon/Nikon, but not Optically Stabilised for Pentax/Sony(or Olympus, if it exists)

Take many shots using both kinds of systems and the one that has a higher keeper ratio is better.

The argument that all lenses need to be stabilised over and again kind'a holds water a little in that, Canon and Nikon produce very cheap 'consumer lenses' in vast quantities with both VR(or IS) and non VR(or IS) and the added price seems to be about $100 or less, and the weight disadvantage is somewhere between 30g and 100g at most.
Technically there is no real(quanitifiable) cost advantage/disadvantage to either system. There are a few benefits in having it in the lens(as far as the future can tell) because there seems to be a notion that you can stabilise the lens on three axis(axes??) rather than just the two of horizontal and vertical, Canon have recently released a fore/aft stabilised macro lens.

Non super tele fast primes with VR/IS are a very rare breed as I see it ATM, with neither Nikon nor Canon offering say an 85mm f/1.4 with VR or IS(yet).

So far, all the evidence points to incompatibility issues in having in body stabilisation AND in lens stabilisation, so if Canon and Nikon produce stabilised bodies they'll almost certainly have a system where the in camera and in lens systems don't/can't clash. No reason for them not to produce stabilised cameras at some point in the future.