Awesome keyboard! I own two: One full-time at work, and one occasionally at home.
Reasons why:
1. Before we get into "OMG it has ABS" - I prefer the ABS on it as it is easier on my hands over a 8-16hr workday. The audio demos on youtube are close but are way over boosted compared to real life.
2. It feels ever so slightly more crisp to type on compared to the Realforce; although I still give a slight nod to the 660C.
3. I don't have my 104 in my vicinity but going off memory the TypeHeaven feels slightly lower profile which is nice. I think it's the lowest profile of all Topre boards. This is nice (if like me) you prefer typing without a wrist rest.
As far as how I rate Topre:
Topre Typeheaven > 660C > 55g