It depends.
To be honest I have the feeling Blu-Ray didn't catch up the way DVD did. And in the future surely there will be a successor for whenever 4k HD video becomes mainstream, but then... I think as of lately with the internet speed increasing all the time and netflix and all other services... the future is likely to be the Cloud (we're living it now, but I'm just saying, things have moved slowly over stream/download side rather than pop in a disc).
If you just wanna watch Blu-Ray (READ) then I think you should go for it, I think last time I checked there were some Blu-Ray R USB drives with DVD-RW capability for around the same price as just DVD-RW.
But if you think you might wanna burn movies to a Blu-Ray (for instance if you shoot video in HD and wanna cook your homemade cheesy films

, or author, as professionals would say), and you have the extra what, $100 bucks? Then... go for it, but I think even so, hardrive space is more efective plus having 25/50 GB worth of info on a disc (in case you would wanna use them as backups) is almost a no go for me, thousands of things can go wrong and I prefer a harddrive-oriented backup solution (or cloud if you have the $$$ and the will).
Obviously, when comparing prices I'm not even having in account Apple's unit, IIRC it's around 80/90 bucks? I'd go for a Samsung one that runs for about 45 on Amazon last time I checked (I'm in the process of buying a Mini so I wanna buy the extra things too).