kowloonmum - I'm not yet a current parent but I've got lots of friends with DS's there, and I've just been through the process with DS, age 6, who has got a place starting in September '11.
After visiting all the schools locally, we thought it was the right one for him (as did his current pre-prep) but we had a back up place at the Dragon in case MCS didn't agree!
The tests were pretty gruelling - he was there for 5 hours, and did 3 1 hour written exam papers during that time. He enjoyed them, and all the boys that I knew who got in, like my son, found these tests quite a fun way to spend a Saturday
. Then back a few days later for class based activities and interviews.
The assessments being so exhaustive is a large part of what makes them expert at picking the boys who are best suited to the education they offer. I don't know anyone whose been kicked out, but there are occasional boys who don't make it from junior school to senior school.
You're unlikely to get the Dragon if applying now. We applied in 2007 for 2011 entry and only just scraped a place (which we're about to decline, oops.)
You might want to look at New College School as a good "back-up" in case assessment day doesn't go to plan (it obviously partly depends on who else applies that year, as at least double the number apply that they have space for).