I think the fact that only around 10% of users are "active" as such paints a picture of:
SecondLife is a great concept, looks interesting and is wanted by masses as such, but to do anything interesting on secondlife you need real $$$ and i cant justify that cash per month on playing an online version of the sims, as its pretty similar aint it.
I didnt bothering registering when i looked, Sony has there version coming out that you can access from a PC or PS3, may just be the ps3 i cant remember.
And if i read right, Microsoft have a vesion in the works for Vista and X360.
But this type of platform is pretty common in some ways, as theres runescape and a collection of other online rpg's.
Its great fun to kill an hour with 2nd life, but after that, you need your wallet out to carry on having fun...