Ya, it's to bad the Pentax/Sony/Olympus has the image stabilization (IS) system in the camera body. You can't see the IS working in the viewfinder like you can when the IS is in the lens (like Nikon and Canon), so you don't know when the image stabilizes so you can release the shutter.
You can tell when Pentax Shake Reduction is ready because there's a logo that appears at the bottom of your viewfinder. I don't mind too much that the image in the viewfinder does not stabilize. Seeing shake reminds me to settle down and use good holding technique.
In-the-body IS also limits the amplitude of movement the system can correct, and of course if it quits working you're just SOL until you get it repaired. Mirror Up doesn't help a lot if some other sytem is still moving around right before the shutter opens.
A lot of people in forums use multiple brands. Common lore is that in-lens has a stop or so advantage with long telephoto lenses. OTOH, all lenses are stabilized with in-body systems, and you only pay for stabilization once. The best of all worlds is to have a choice. SR can be disabled when it's not needed, but I only turn it off when using a tripod. If you don't have a stabilized body or lens, then you don't have the choice. There are stabilized third-party lenses available for Pentax, in which case you have to disable one stabilization system or the other. Most people report no difference except possibly with super telephotos.
I don't know that there's any greater risk of failure from in-body or in-lens systems. One thing I do know is that I keep my bodies a few years, but I intend to keep my lenses forever. Stabilization in the lens would be more likely to fail, because of longer service. Which brings up another advantage of SR. Pentax has improved their system twice. The K-5 for example has double the range of motion compared to a K-r. It also compensates for rotational motion, which neither the K-r nor in-lens systems can manage. It is likely that improvements will continue to be made, and will benefit every lens.
What you said about the sensor moving during the exposure is incorrect. Sensor motion starts with the shutter action and stops before the exposure starts. Here's the patent that explains it:
Application # 2008/0226276. ANTI-SHAKE APPARATUS - Patents.com
Nikon hasn't changed their mount since 1959, so most all Nikon lenses made since then will work on any Nikon film or digital body. About the only exception to that is a couple of the specialty Nikonos underwater cameras. Of course, cameras in 1959 had virtually no electronics so though old Nikon lenses will work on today's cameras they may not meter, aperture may need to be set manually, and they may not auto focus, but if someone has a good grounding in the technical aspects of doing photography, none of that's much of a problem.
There are different levels of accomodation though? I believe the D7000 for example has better compatibility than the D90, and both have better compatibility than the D5100 and D3100.
Sorry if I'm a little ignorant of other brands. I only know how Pentax works. If I use a fully manual lens in Av mode, it will expose properly but the aperture will be wide open. If I use M mode, I mechanically move the aperture to set f-stop, and then I hit the "green button" and the camera will meter (i.e. the camera will automatically set the shutter speed for proper exposure). Do Nikon and Canon have this feature or something similar?
I'll be away from my computer for a few days. Hopefully we can catch up later. It's been informative and pleasant chatting with you.
