You'll pretty much know exactly how good the expansion will be before the launch. The beta offers everything the expansion has to offer, and in the next few months with the tweaks and official Blizzard statements we'll get a crystal clear picture of RoS as a game. This is a fair move by them, letting the players watch/play through all of the content in beta and decide for themselves is this expansion really worth their money.Quote from PietrakI don't believe anybody who says "i'm not buying RoS because it's too expensive". I just don't. We will all buy it just to see how good it is.
Considering the amount of hours of gameplay a new class, act and adventure mode will offer, 40$ seems like a good deal. If you simply enjoy farming, maybe trying some fresh builds with the new legendaries, than RoS is definitely worth the money on launch. All these aspects of the game are polished and work great in the expansion, along with the streamlined leveling system.
Other than that, the game doesn't really offer much, and it seems that it will sadly once again get pretty boring after a couple of weeks (mostly for softcore players). So, if it does feel like 40$ is too expensive for you at the time of the launch, and you have better things to spend your money on, it's OK to wait a bit and see how everything will play out in the weeks after the game is released. Even when, or if, ladders and competitive PvE are released, the players who got the expansion on day one won't have any real advantage over a player who bought the game later on.
Better to wait than to get frustrated for spending money on something you won't really enjoy that much. This time, Blizzard made the right move to make all of the content public months before the release, and no one will be right to blame them or to ask refunds for the game like they did with the original D3. RoS will mostly sell copies on it's own virtues and quality of design, which is a good thing, while D3 earned a lot of millions solely on the legendary legacy of the Diablo franchise.