i did the assessment a few weeks ago and was really really impressed (am joining in September!)- 2 other CDCers were with you throughout the morning, really supportive and all felt quite laid back.
The verbal reasoning test is fine - you have to work quickly though but is easier that the ones you find on SHA website.
The group exercise: you will be sat in a small room 4-6 people with 2 capgemini people watching. You are then given 15mins to read loads of material (flick tho first to find important bits!) and then 30 mins to complete tasks. This isn't too bad - make sure you say things that are constructive and coherant - not just for the sake of speaking!
The partner interview is quite tough - this is where you discuss your prepared business case and also do on the spot business case. Just make sure you keep calm and think thro your answers before speaking.
Apart from that enjoy the day and good luck!