They are getting cheap for a reason. Well, for several reasons, including that BlackBerry are currently doing a bit of a Nokia in watching the future disappear over the horizon without them, and the Playbook doesn't offer anything uniquely positive but it does have some significant downsides so it isn't selling.
IMO, they are good as long as you already have a BlackBerry to which you can tether them, but pants otherwise. That might be obvious but you don't, by way of comparison, need a iphone to make an ipad work.