As you've had no responses thought i'd chip in here.
I didn't send my daughters there - but I did go and visit, and by all standards the teaching is wonderful. What put me off was the travelling to Kings Cross daily (bus provided - assume you're Belsize Park-ish), and now the school is looking to move to part of Haverstock School (which I find hilarious as probably zero of the parents will send their kids to Haverstock).
This disruption with a lack a permanent home was an issue for me - it may not be for you. There will be parents whose kids have gone all the way through Abacus who assumed they'd be permanently settled in Hampstead Police Station the next year - quite simply don't assume your child will ever be in a permanent school building!
I loved the international aspect of the teaching (Chinese taught for example), but make no mistake Abacus is where many parents nervous about 'state' but unwilling to go 'private' send their children.
There are some parents who like trusts / free schools - I think it is the only one in Camden. But there are falling child numbers in Camden - so you'll find numerous wonderful state options available to you that weren't for me 10 years ago (Camden has had to close 2 primary schools due to falling numbers in the couple of years).
Many parents also are not so keen on the more liberal ethos of Camden Primary Schools and I respect that (I love it though) - so again Abacus may be an option.
For more current information on Abacus (if this bump doesn't bring any info to the fore) then join 'Hampstead Mums' on Facebook where Abacus is a regular topic of conversation - you'll find dozens of parents on there willing to tell you how wonderful it is!
Most importantly, go see it - and then go see another couple of schools that have places - New End, Hampstead Parochial, Fitzjohn's, Gospel Oak, Fleet, Primrose Hill and so on (depending on where you are of course). You're lucky you should have a choice of great schools - nothing in Camden is poor (or if you prefer 'inadequate' by Ofted standards).