My 2c worth as I've just stumbled upon this thread.
I've stayed more nights at the JW in Bucharest than I care to recall and made Gold on the back of 50+ nights a year in this property.
Some comments:
Rooms
1. When you check in make sure you ask for a room near the elevators. Otherwise you could end up a veeeery loooong walk to your room. They'll know which ones to offer.
2. If you have booked an executive room they may try to stick you in a room far from the elevator on the 8th floor. Avoid this if you can as the rooms here apart from being far also tend to be small. Ask for an executive room in any of the other floors which will be much better, close to the elevator, and very spacious
3. Avoid rooms on the 2nd floor. The noise from the ballrooms and the sports bar on the 1st floor can keep you up late at night
4. Avoid rooms with connecting doors as these are thinner than a sheet of paper and let through all sounds. If you get noisy neighbours you'll be able to hear every conversation, their phone calls, their tv.
I agree with what other said about the gym, it's pretty good.
The restaurants in the building are varied:
1. The Italian (Cucina) was once considered one of Bucharest's best restaurants. Food is still good there but service is awful. It's slow and unresponsive.
2. The Sports bar is exactly what you'd expect. If there's a big sports event that day make sure you book a table well in advance otherwise you won't even be able to get in
3. La Cupola is a very generic restaurant in the same space as that used for breakfast.
4. Lebanese restaurant. This is excellent. It's in the Marriott building but separate from the hotel so you won't be able to charge it to your room
5. Apparently there is a good but expensive Japanese restaurant now as well.
Taxis
It's true that these can be dodgy. The limousines outside the Marriott can be expensive but if you ask the bellboys they'll usually be more than happy to call you a regular and safe taxi. No need to cross the road for one.
From the airport, you can get a taxi from the taxi rank outside the building. There will be a long line of them. All belong to the Fly Taxi company and they charge abou 3.3 Lei (~1euro) per km for a total to the hotel of about 60 RON. If there is traffic (anytime between 7:30am and 8:00pm) it will take up to 1:30 to get to the hotel from the airport. Bucharest's roads are awful. At other times the hotel is 20 minutes away.
Hope this helps.