The back protector built into my Dainese leathers is really unnoticeable and doesn't move at all.
For the Goretex jacket (touring/raining/short trips thing) it doesn't have a protector built in so I wear one with arm straps and a kidney belt. The only problem with this is that it tends to ride up your back unless you have the belt done up really tight, which then becomes uncomfortable.
Don't know what the ones that have a strap between your legs are like, but I'm sure they do a pretty similar impression to cheese-wire down there if worn for any length of time
