I like that the center point of a prim changes when you make it flexible, but I'm noticing that the center-point change when you make a prim flexible is inconsistent - some parts of SL seem to still be using the old centerpoint.
Make a .1x.1x10 prim, make it flexible. Move the camera around then hit 'H' (with the chat bar off, of course) to target the camera on it. You're probably targeted on empty space now. Turn off flexible, notice that you're now targeted on the center of the prim.
Make that prim flexible again, stick it on your head. It's probably floating way out in front of you. Select it, turn off flexible - it's rammed right through your head, with the center point right behind your eyes.
Linking long flexprims is also problematic; it's much more finicky about distance than normal. In my testing, I got a lot of 'link failed-pieces too far apart' errors when trying to link long flexprims to non-flex prims: rez two prims next to each other, make one 10m long and flexible, move it up so that the flexprim centerpoint is near the static prim. Won't link.
(Mac preview client.)
Also, I can think of all kinds of crazy things I could do if there was something similar to the 'target' functionality that particles have - hide your arms with invisiprims, put an arm-length flexprim on your shoulder, and aim it at prims on your hands for a 'rubber-hose' look like an old B&W cartoon character!