They will have 4 mln subs in 2 years , not 5. In 5 years they will have some 15-25 mln subs (at least these are current estimates). They will be a money making machine.
The service is great and people love it. It beats CDs, MP3 and IPODs hands down. People will realize that sooner than later.
Valuations is a different story. There is currently alot of excitement about Stern/Karmazin and as any story stock it is prone to wild price swings. But current valuations are reasonable if one assumes that they will have some 20 mln subs in 5 years which will result in earning $1 bln a year or so. So there is still upside left for years to come.