Here is a thread that you might find helpful:
http://www.flyertalk.com/forum/showthread.php?t=136153
While that thread was started about 6 years ago, it has posts as late a May of this year (2006) and it has a good variety of reviews to help you. Also, if you are not "stuck" just on Marriott alone, you might also look around the Singapore forum. I would think there would be a lot of questions/answers there about various hotels in Singapore and their various benefits.
With that, I tend to make it to Singapore once or twice a year for business and I normally stay at the Marriott. Yes, the comments made that the rooms can be a little on the small side are accurate, but overall, I have no problem with the rooms. I personally think this Marriott has one of the better overall lounges in the Marriott system. They tend to have a wider selection of foods than normal, it is open 7 days a week, and even throughout the day. And as you have mentioned, its location is one of the best if you want access to Orchard Rd and to the subway system.
I guess it would help to know what exactly you are looking for? Is this a business trip, pleasure, honeymoon? Are you looking for great views, large rooms, extra services, good lounge, to use or recieve Marriott points?