123 Doc is an excellent resource. It is harder than onexamination. I used several websites and they all have their benefits.
The tutor interaction on onexamination is far superior to 123 Doc, also the spread of questions is strange with almost 5 times as many neurology questions as dermatology or nephrology. The top ten is good for motivation, although I would like to see a raised threshold for the minimum number of questions answered.
All in all though I would recommend it to anyone studying for their part one.