As an expat in Bangkok, I feel that those who have posted about getting ripped off in MBK have been very unfortunate and probably bought from small stalls and not larger, dedicated camera shops.
MBK is the best place to go for cameras. It is better than Pantip Plaza and Fortune Town.
There are at least 4 very good, dedicated camera shops at MBK and I honestly feel none would rip you off.
Foto File on ground floor, Big Pro Camera, Foto Thailand, Sunny's etc are all legitimate shops and I buy all my stuff from them without issue.
The poster who said "Go to Hong Kong" is silly. You'd not pay to go to Hong Kong just to get a camera, so his / her comment is only valid if one is already there or planning to go, BUT you're MUCH more likely to get ripped off in Hong Kong and prices fluctuate there by hundreds of dollars leaving you completely unsure of where to buy.
MBK is a good place to get your camera equipment. Prices might be higher than HK or even Singapore, but with a VAT refund things might work out OK.
For more information, you could always go to:
http://www.thebangkokshoppingguide.com/where_to_buy_a_camera_in_bangkok