I read this forum with interest. I am a CX Diamond card member. I fly CX because I want to get there in one piece - CX pilots and maintenance are good but the cabin service varies widely, usually a sign of how good the I/E Manager is (but still far better than anything in USA). After zillions of flights you get to know what the cabin crew are supposed to do and notice when they do not perform the set routines.
Before you go and spend zillions, depending on where you are based to become a Diamond member, think of this:
I have been a Marco Polo member for 19 years and have yet to be invited out for a drink or meal by the MPO club
Half the IEM's check their Pax list and introduce themselves to you and Gold members, half do not bother.
When you get to your destination on the tarmac , there is no special bus for First class pax provided so you are in with the plebs in the run to immigration.
Certain airports eg Bangkok have no First class checkin so you queue with the rest.
Previously the Diamond card was valid for a couple of years - now you have to requalify every year.
Before you became a Lifetime member after flying 2 million kms - now it is by invitation only.
After you achieve the 120,000 miles your card is sent to you thru the post stuck on a blank card with a piece of blue tack - no cocktail party, welcome phonecall, upgrade certificates, proomotional gift or the likes. (Unless you live outside HKG in which case they actually entertain you and write stories about you in the CX Rag)
Before, Gold card members had their own section of the lounge at Kai Tak - this disappeared under One World.
Before, you got a choice of gift when you reached the next tier level. Now they have done away with that gift stating that members told them they did not value them (Bullsh-t for penny pinching)
CX spent thirty years building up the Marco Polo Club and have denigrated it in the time since One World appeared.

If you are chasing the Diamond because you want the best inflight entertainment and service go fly with SQ who innovate and whom CX poorly imitate . I cannot remember when I last got an upgrade since so many of the CX planes no longer have first class except on long haul routes. The SQ 777's have 12 seats in First Class, CX has none.
On the Plus side:
They will bump you to the top of a wait list if you change your flight itinerary.
You can have a sit down meal in the FC lounge served by Peninsula staff at CLK but be careful you do not stand in the water trap that they have reinvoked.