We've stayed at the Hyatt Erawan many times over many years - it consistently delivers great service.
The hotel facilities are good, the food & restaurants (all of them) are great & relatively well priced for a big business hotel.
The spa is also very good, although it is quite expensive for Bangkok. The pool area is also lovely.
The rooms are what you'd expect in a business hotel; although a couple of times we've been caught out with king singles - the tip is to specify a king size bed when you book.
The location is great - very central for all shopping & near the sky train for easy transport all around BKK.
Concierge is very good, and has a list of restaurants & spa's and will willingly book for you. Although the limousine to the airport is prohibitively expensive - so only do this on a business account!
The meeting facilities - especially in the 'Residence' are first-class - the food you get here when you have a meeting is not only plentiful, but also very, very tasty. be prepared to put on a few kilos during your stay.

I have been staying at this hotel for the past 6 trips to Bangkok. It has everything, great food, great location, great service, great night life, great car service from the airport and best of all a great staff. Everyone I have met over the past few years is still working at the hotel…its truly incredible the small amount of turn over in the executive floor staff. I will stay at no other hotel in Bangkok. The location is right next to the sky train, it’s the only way to get around Bangkok. Let me give you an example of there service…on my trip before last my laptop came up with the blue screen of death on day one of a fourteen day trip through Asia. After consulting with our IT at home in the US it was determined the computer was dead. I took the laptop to the concierge desk and simply asked for help. The packed the laptop, shipped it back to my company and directed me to three computer stores to look for a replacement. It was a bit confusing and I was not comfortable with the deals I was seeing…upon hearing of my concerns one of the staff accompanied me to the store, after she got off work, and negotiated me a great deal on a new laptop…much better than I could have received. These folks will go the extra mile for you!

Stayed there 4 nights with my wife, we arrived early in Bangkok, check in supposed to be at2pm but we were there around 9am, it took them 45min to prepare the room and we checked in that early without extra charge or single comment from hotel staff (some other hotels tell u off if u arrive that early!!), reception has a tropical atmosphere with trees and water running around which was nice,
Hotel staff :extremely polite-smiley and very helpful,
Rooms: as any 5 star hotel, with a Thai decoration, seperate shower-bath
Location: great location in central bangkok, close to all shopping centres, next to sky train station (chit lom),walking distance to silom park-MBK-Siam paragon etc
Breakfast: variety to choose, starts 6:30am
Tours: they do organise privite tours, our guide was there within 15 minutes!! reasonable prices (floating market- siam niramit, etc..)
View: from our room (12th floor) we could see silom park

1 thing i did not like: you have to pay to access internet from your room,i coul pick the signal up but u need to register and pay to use it which was bit odd for a 5 star hotel, there is a computer room in the lobby but again u gotta pay to use it

overall: if you could afford it then go for it, u wont regret it.

I stayed at the GH Erawan for 5 days as part of a conference that was held there.

I found the stay to be excellent and up to the Hyatts group standards. Some of the points to note are:

Location:

45 minutes from the airport and connected via an overhead walkway to the BTS skytrain station. The same walkway takes you to a number of shopping mall i.e. Amarin plaza, gaysorn plaza, central world etc etc without you ever needing to get down to the street level. As other posters have mentioned, the skytrain will take you to other closer destinations like siam center, MBK etc.

Business:

I agree with a previous poster, the business facilities are amazing. "The Residence" is gorgeous and the meeting rooms could be mistaken for the plush sitting rooms / drawing rooms in a mansion. There is a large kitchen where we had a team building cook out session. This place is in addition to the standard meeting rooms and halls in any addition. Highly recommended for business meetings.

Customer Service etc:

Typical Thai attention to customer service. Requests promptly handled with courtesy. The breakfast buffet is large but was not to my taste as the variety was more focused towards oriental delicacies and pork dishes. Room service food is pretty good however a tad bit expensive (considering the fact that you are in Thailand).

All in all, this is a classy place to stay in Bangkok and highly recommended.