I've never liked shooting with an IS body (I've been reviewing the Sony Alpha 200). Unlike in-lens IS, the viewfinder shake from hand-holding is still there.
The effect of IS can immediately be seen in the viewfinder for lens-type IS... so composing your photo isn't a nightmare unlike in sensor-shift systems. This is because the IS components in the lens are already compensating for handshake
before it reaches your viewfinder. It's almost like using a monopod.
Composition is easy because you don't have to deal with a shaky viewfinder image caused by your handshake, for the abovementioned reasons.
It also gives you a sense of security that everybody from newbies to professionals will appreciate, because you can see the IS system actively working, as opposed to sensor-shift where you can only see whether you have a keeper or not
after you capture the image. You just have to "trust" the IS to work, and "trust" that you framed correctly with the shaky viewfinder. Here's the bottom line: sensor-shift is less forgiving, especially with the framing aspect.
And I definitely get more stops of light shooting with telephoto lenses >150mm with lens-type IS as opposed to sensor shift IS.
Let's be pragmatic here, shall we? If you want to know why Nikon and Canon insist on going with lens-type IS, I suggest that you pick up an EF-S 55-250 IS (about $360) coupled with any Canon DSLR that can accept EF-S (DX) lenses. Go on, try it out at your favorite friendly camera shop... you won't have to spend a cent to answer your nagging questions.
Like the Japanese say, different strokes for different folks. Sensor-shift IS is invariably cheaper across the board, as manufacturers don't have to install IS systems on all their lenses. One would usually have more lenses than bodies anyway, so it's definitely a more cost-efficient method... especially since sensor-shift works very well on lenses with short focal length. It makes economic sense.
But of course, rule of thumb is... things that makes economic sense usually have a trade-off or two somewhere.