Des Indes is a very well known hotel in The Netherlands (and even more so in Den Haag/The Hague). The Dutch government is based in The Hague and Des Indes is the preferred place to stay for a lot of guests.
You can get to The Hague from Schiphol by train easily. The Hague has two big train stations: Hollands Spoor and Centraal Station (Central station). Most trains go to Hollands Spoor and continue to Rotterdam; some, however, go directly to Centraal and then turn around. If you take a train to centraal, you can walk to Des Indes or indeed take a taxi for about 5 minutes. From Hollands Spoor, it's about a 30 minute walk (wouldn't do that if you're not familiar with the area) or a 10/15 minute taxi ride.
Den Haag is a very nice city, and infact I used to live there for about 4 years. There's a lot to see; museums such as
Panorama Mesdag, Paleis Noordeinde (palace in the middle of the city), and
het binnenhof where, amongst other things, the Tweede Kamer is located (which is where I am as I write this
).
I don't know about Plat upgrade successes but in general the Dutch Starwood properties are very nice to plat's - I always get upgraded to suites at the Schiphol and Rotterdam properties.
Hope this helps!