At Westminster we have a Paxton fob system (installed by the previous wardens, I understand - thank you, Lorraine!) and I love it.
We don't offer codes, but fobs. Each fob can be programmed for particular timeslots on particular days, and easily reprogrammed on the system without the fob itself having to be present. We can cancel the fobs remotely when they are reported lost (and we have a deposit system to help encourage people to look after their fobs). You can also look up in the system who has used their fob when, should that ever become necessary.
I much prefer it to the newer code-only system we have at Hammersmith MH, which seems a lot less flexible.
Please feel free to give me a call to discuss it further, if you want to hear me sing Paxton's praises in detail!
all my best,
Lorna.