Blu-Ray has been beneficial as proven by 7.1 sound, multiple languages, and more. A faster Blu-Ray drive would be even better, but that would have just jacked prices higher during launch. Obviously it's downside of being slower (than needed) has caused devs to duplicate data, or install to HDD, but it's benefits have clearly outweighed the downsides.
As for the Cell, I feel it is a wash. They could have made a system just as powerful using a simpler architecture, which might have prevented some of the really sad ports and large number of complaints from developers. So while they have had some technically impressive games with the Cell, it wasn't necessary. Maybe in the future my opinion may change, but who knows.