Well, here are my two cents on the whole process:
I applied for straight Maths, so I had (essentially) 5 Pure questions to answer in the exam. Back in the day (i.e. December), the test was based on C1-3 knowledge, but I found that some Further Maths concepts like roots of equations and formal proof by induction helped me in part. Now it's based on C1 and C2, since it's being sat earlier. The only thing I would stress is that you know these two modules inside out for the exam. Looking at the specimens is also advisable - just to get a feel for the structure and style. And that by the time interviews come, you should probably know C3 to a similar depth, along with C1 and C2. Any other Pure modules and a good knowledge of elementary stats could well help. By the time interviews came, I'd already sat C1/2, M1, S1 and FP1 (in June), and was about to sit (in January) M2, S2, C3 and C4. Questions in my interviews pooled knowledge from all of my pure modules in some form, but mostly the Core content, and from S1.
About the actual test, I was told that I got 85%, a respectable score for this year - the last question on this year's test was (according to a tutor) 'too easy', and so raised the average of those gaining offers up into the 80s. This, of course, means nothing for the future cohorts, as your tests could be easier, harder or just the same difficultly - what I think is appreciated is clarity in the 4 longer questions, which is why I aimed to quickly, but not hastily, finish the 1st (multiple choice) question.
As for interviews - I applied to Merton, and was assigned St. Hugh's as my 2nd choice college (the department assign 2nd and 3rd choice colleges as standard, and you're guaranteed an interview at 1st and 2nd choices). I had 2 interviews at Merton on the first day, the first of which I came out very happy - the two interviewers, Dr. He and (I think) a postgrad student of his, were very friendly, and started interview with some pointless but comforting chat about some books that I'd read. The maths itself varied from thought-provoking to algebra-crunching to nasty in every sense. The second interview, in which I made a lot of ridiculous mistakes (hence coming out mildly distraught), was similar in structure, was taken by Professor Tillman and another tutor whose name has temporarily slipped my mind - they made me feel at home on their very comfortable sofas, but had little time for chit-chat. The questions seemed to be more logic/almost philosophy based - I'm not really sure if this difference was intended, or whether it was just the tutors' pick of questions. Both interviews had some combinatorics or probability, both had some calculus, and both had graph sketching, suggesting that these are important skills that the tutors are looking for, so perhaps practice these.
I then had a free day on day 2, and an interview at St. Hugh's (what a way away it was!) on day 3. That interview was entirely based on one topic, but progressed in difficulty, as had the other 2 interviews. It's probably a good sign if you are set a really nasty problem, as the interviewers wouldn't set it to you if they didn't think you could do it, even if with a little hinting, prodding or shoving.
All in all, just make sure you know your stuff. If you don't, you might not pick up on a hint, which can leave you stuck on the same problem for aaaages. If you haven't covered a specific topic they set you, do tell them, but extra knowledge is always (obivously!) good. Make sure you talk in the interview. A constant silence whilst you think is not helping you or the interviewers. Say what you are thinking - tell them about how you arrived at your answer, or why you think your method is better than an alternative. That really will help them help you go in the right direction if you are stuck, as well as helping them to gauge how good you are, and could clarify the problem for you. Lastly, everyone needs a bit of good luck, so here it is - good luck!