Its praticly a second life, you can even buy you own island if you pay $1600 for it.
Gameplay: You can fly and walk, but can't swim, also can drive cars, airplanes and many other cool vehicules, but you have to buy them first (a car costs aprox. $3). There are many user plans form Linden Labs (The developers); with the basic / free acount you can practicly just walk around, chat, look at objects and nothing more, with the premiun plan you can own land, yes, virtual land has its own cost (if you are intesting visit secondlife.com website and see land prices), only if you have land you can build a home or a bussines.
Graphics: The graphics are good, you can customize you character face and body and clothes have also a price, so you can't change your custom unless you pay for new clothes. Sometimes ambient textures don't load well, but still the game looks nice.
Sound: The cooler stuff, because you can hear as background sound music made by the users in the game (I don't know how you can create music, maybe there are music studios in the game), but the other sound fx are poor.
Value: Its praticly a second life, you can even buy you own island ($ 1600), but you have to pay money for it, so, I prefer to spend my money in real life things like real food or real clothes than in a virtual object that just my virtual avatar can touch.
I don't like it, but everyone has its own opinion, the game is free to download and install, but you should start paying money for objects and land if you want to have some fun.