A couple of girls from my DS's class are going in Sept and a couple applied and did not get in. As 7+ was the main topic of conversation at the school gates (DS is a pre-prep so lots do 7+ for various schools), I have a bit of knowledge on this through the other parents (I paid particular attention to discussions on City as I will look at it for my DD in 2013.) (I have name changed for this as facts might be identifiable)
I dont get the impression that the test format was particularly different from other schools like South Hampstead or Highgate. The main problem is that City have moved it forward and it is now in November (used to be January, as for the other schools). This can be hard for the younger ones in the year who are less likely to be ready emotionally a couple of months in to Y2. Also, for the girls who dont get in (about three quarters of the girls that apply), because all the other 7+ schools in the area have the tests in January, I think it becomes a really long drawn out process to have to repeat the whole thing in January.
Of the 2 girls who got in, my impression is that one was always a dead cert (extremely bright, good across the board, future head girl type). The other is also very bright but much less of what I would think of as a typical City type (a bit of a dreamer). Of the 2 girls that did not get in, I would say that they both were considered to have a pretty good chance and one in particular was definitely expected to get in and was quite a typical City type (but both got into very good schools in the January process - one is going to South Hampstead and one Highgate). Funnily enough, the girls that got in seemed to enjoy the process more (apparently they both said it was really good fun, particularly the second round which is more like a team building exercise than an interview). Maybe the format just suited them better. One of the girls who did not get in took it quite badly.
I think about 100 applied and they invited about 50 back for the team assessment day. They have around 24 places. Based on the girls I know, there are probably more than 24 who would be capable of getting in, so there is a bit of luck.
Having been through 7+ with my DS at different schools, I think the important thing is to keep it as low key as possible and make sure they do not get fixated on a particular school or to feel they are being judged. We told DS that it was about the schools getting to meet him and that once he had done the tests we would discuss with the schools which school we thought would be best for him. Had he not got an offer from somewhere, we would not have told him (we would just have told him which school he was going to - we had a non-competitive entry school as a back-up). Unfortunately, i think it is a bit harder to manage with City which is why one of the girls got a bit upset - they got the bad news from City before they got any good news from anywhere else (and I think her parents had been extremely keen on City).
Also, I would only take horsemadmom's advice about not doing prep if your DD is at a pre-prep that prepares then for 7+. Schools like Hampstead Hill send a lot of children to the competitive entry schools at 7 and their reputation turns on getting kids through (apparently there are about 4 or 5 going form there to City this year). Hence, they do a lot of prep in class. Even at DS's school (which we did not think of as particularly pushy) they did some practice papers in class (VR, NVR, maths and english) and practised extended story-writing, so if your DD is not at a school that prepares for these tests it is worth trying to put your DD on an equal footing with the pre-prep kids by buying and working through some Bonds papers - I dont think it needs to be loads as it is more for familiarity than anything else. Some of the schools also publish sample papers on their website (e.g. Highgate and NLCS) and it is worth having a look at these even if your child is not sitting for those particular schools to get a feel for the level. DS enjoyed the practice papers, esp maths and reasoning (less keen on comprehension)
I dont think City do publish papers but they should say on their website what they cover in the tests (eg some just do English and Maths, and some also do reasoning, most do reading and spelling).