I've got an Icon Field Armor vest
http://www.newenough.com/armor/back_...rmor_vest.html
The kids call it my armadillo - it's got hard shells that overlap on the back.
I did NOT like the soft-foam type - I can just see my spine getting messed up if I landed on my back... the Icons hard shell protector is shaped with a bump out so your spine bones should not get touched at all (in a relatively normal crash landing anyway). This is the MAIN reason I selected it over others (I have a good friend in a wheelchair from spinal injurys).
I really like it except that the sides of it are velcro attachments (my version is anyway). It's OK under a leather jacket... but I see people riding with these over a tee shirt and there is NO WAY it would stay on in a spill (not mine anyway with it's old old velcro). I'm going to sew on snap attachments for the short term some rainy day.
edit: I commute daily wearing it, and have no complaints about it. Once the hot Summertime hits - it may be a different matter altogether!
- Carlos