Though the Dutch are a kind of tricky civ with its abilities, I think people do underestimate them.
The idea of the UA is maintaining half of the original happiness from a product that you have gained. This means that, if you trade your 1-stock of gems at another civ for his 1-stock ivory, you end up with +6 happiness. Or you can trade your 1-stock for other sources but can still balance out your happiness better than other civs. This also means that as an AI the Dutch will be more willing (and perhaps less demanding) to trade its last stock, making them a nice trading partner (though this depends on how they will put together the Dutch AI).
There are two weaknesses in that ability. The first is that during time, when civilisations expand more and gain possession of other luxuries the Dutch ability will become less valuable during the game -> this might be balanced out though with the new added luxuries.
The second is that it also means that for the Dutch to benefit the most out of its ability it requires to expand enough to different territories with unique resources.
Interesting note: though the Dutch benefit the most at having peace with other civs, this does not count for CS. We've already seen some pics of warmongering Dutch against CS, and because of the new mercantile states it might be likely that the AI goes on a conquering spree against them. So if you have an mercantile state as an ally and the Dutch are in the game, keep a close eye on them.
Polder: I am not sure about its abilities yet, but it would make sense if they can produce tulips. If those are being added as a new resource (after all, the Netherlands exports about 80 to 90% of all tulips in the world). Depending on the settings that you've set, marshes aren't so much common that it would allow the Dutch to gain huge amounts of tulips. But we have to wait and see about this, its still a tricky thing.
Sea beggar: not much known about either. As far as I know it is an improved privateer that can also capture other ships. I'm guessing those would be embarked units, that would make sense to me. They sound powerful but this is again something that we'll have to wait and see.
It remains difficult to make some good conclusions about the Netherlands, but we can say that the Netherlands has its liabilities and is dependable on several factors. But if played correctly, they'll be a good civ to play with. They are probably going to be a civilization for the more experienced gamers.