I've been a regular at the British Library for many years. I even a had a readers's ticket when it was still based in the British Museum. But until now I haven't used the Indian Office collections, but a client recently asked to see whether there were any files about an Indian politician active at the time of Partition.
Like the rest of the St Pancras site the room is sumptuously furnished, with book cases around the walls full of reference works and catalogues. Not much infomation is yet online (although there is some stuff on Access to Archives A2A). It is quiet with a separate area for microfilm readers and a security guard on the door checking tickets on the way in and bags on the way out.
It was reasonably busy when I was there with a mixture of academics, professional researchers and family historians. The staff were extraordinarily patient with all enquirers and very helpful, particularly with the convoluted and bizarrly arranged ordering system. Files and books take about an hour to 70 minutes to arrive,
Room: 8; staff: 10; resources 7; ordering 2