Yes, pluses and minuses to a subscription service - clearly if you still have all the older consoles, then this won't bring much benefit to you.
Remember though, this will bring games from all the previous Playstation models, not just the PS3 - useful if you want to hark back to the days of the original WipEout, Ridge Racer, Resident Evil or Fade to Black. Also, it will be available to a whole host of Sony devices - allowing you to game on the move with your Vita / Xperia phone / tablet, and on some of the TVs as well, on the same subscription and without a console (though you do need the DualShock 3 or 4 controller).
Will need to see how much it is, but this represents the first step of gaming following the current trend of subscription streaming services, like we're seeing with music and TV / films.