flat screen wall mount question

I would like to mount a Samsung UN46B8000 46" that weighs about 35-40 lbs. on one of those wall mounts that tilts, swivels, and extends. My question is, for the people that have similar displays mounted like this, do they wobble around, when repositioning them, or are they smooth and fluid? Or if somebody just touched it (Heaven forbid!) or bumped it, would it shake? I don't know of anyone personally using one of these, so I've never seen one "in use". I think I'd rather just mount it solidly to the wall (with no tilt/swivel/extend properties) than to have it all "wiggly" when moving it.

And do they loosen up over time? I would hate installing one and then have it "fatigue" after a while.