I've stayed at the Marriott Curaçao on numerous occasions due to an extended project on the island, and have also tried several other hotels at various times - both upscale and not; the Avila, Hilton, Clarion, even the Plaza in downtown Punda. All in all, I return to the Marriott whenever the budget permits as their seasonal rates get pricey. Admittedly the main reason for this is Reward Points, but also it is consistently the most comfortable of the viable options.
If you have the opportunity to stay at the Avila, it is extremely modern and posh (for the locale) but again the high-season rates were pushing into the Manhattan price range.
Besides points, the main attraction is the Marriott standard. After working all day in a place most people go to play, it's nice to walk through the open air lobby and get a chance to enjoy where you are. Remember that you are in the Caribbean but Curaçao has that Dutch efficiency, so rooms and facilities are up-to-date and very well maintained; decent shower with hot water when you want it, beds that are as good or better than the one at home, wireless internet that works with VPN (not cheap but reliable), decent workout facilities, gorgeous pool/beach area (nicest of all the hotels that I've seen), snorkeling right off the beach and the business center has a lending library if you forgot to bring something to read during off hours.
My first time there I was still Gold Elite (can't always pick Marriotts) and I did in fact get an upgrade to a beach-front room. Most of the time upgrades are not available, even for Platinum members, but the Garden rooms (ask for ground floor) have patios that open up to the grounds and let you walk right out to the pool or beach areas.
Food is expensive in the restaurants, but a good option if you don't want to travel into town where the selection is pretty good. If you don't have a car and are taxi-bound then you're stuck with expensive resort food. One bad experience was ordering the sushi in the bar (I know... bad idea from the start but I was tired), the fish was fine but the rice was so old it was crunchy! They did bend over backwards to fix it once I complained, though.
All in all, a great place to stay on business, and I'd consider it for a holiday if I wasn't already tired of the island - it's very small and I've been going there for almost a year.
Highlights for free time are: Thursday nights at Club Cinco (ask...), several full service beaches - some lazy and uncrowded, others in perpetual Spring Break, dinner and the view at the Boathouse restaurant, sitting in a micro-version of an Amsterdam cafe in Punda.