Obviously a different travel experience, but years ago (2010) in my early 20s I spent 6 months flash/backpacking from Beijing to Singapore. I visited Halong Bay on this trip and it was one of my least favourite spots - the amount of rubbish and stuff floating in the water was unbelievable. To be honest, if you've seen the karst landscapes around Krabi/Phangnga Bay or Guilin/Yangshou you've already seen what Halong Bay is about (I saw all three on this trip and much prefered China/Thailand). Even after a more recent visit, I'm not a huge fan of Vietnam in general.