poc spine protectors are pretty nice, and protect well, i've used them, and owned a similar one. they aren't noticeably heavy, but more awkward and a little bulgy, and i always had the problem of the waist belt riding up.
I highly recommend looking into slytech,
http://www.slytechprotection.com/back-protection/2nd-skin-logo-standard/
i got one of these last year, and i wear it without the straps, and just wear a tight under armor over it.
It's really nice, because your body heat makes it really pliable, so you can barely feel that you are even wearing it, and then it hardens on impact. The tail bone pad is super nice, too. And no shoulder straps means no riding up.