For your example of webOS, yes the devices still work, BUT they get no official support, no bug fixes, no security patches, nothing other than what a small dev community comes up with. Blackberry is a serious unknown. Yes, all they announced is that they are going private, but I don't see how that will change their situation that much. They will still have an OS that has nearly no future out side of MAYBE some corperate and/or government use. Not exactly the sort of thing that will have developers rushing to develop their apps for the platform. And as more corperations go BYOD and iOS and Android both get more enterprise friendly, the market for Blackberry will do nothing but further shrink. There is speculation that Blackberry is going private in order to be dismantled and sold in pieces or that the offer was only made to try and attract another buyer for the company.
I for one would not buy a Blackberry until at least a few weeks or a month after the buyout becomes final so we can have a better idea of what Blackberry's future will be. Even then, go into it knowing you are on an OS unlikely to get many of the apps you see your friends and family enjoying. IMO, if you really want to switch to another OS, wait until Windows 8.1 launches and get a Windows Phone 8 and a Windows tablet. There is a much more certain future there than there is at Blackberry.