It's subjective to a degree. Obviously it depends exactly where you are but there's also the factor that only Vodafone and O2 use the 900Mhz band, which is much better at propagating through things than is the 1800Mhz band used by the others (and Vodafone/O2 themselves).
What this means in practise is you'll often find reception inside buildings is better with O2 or Vodafone and my own experience bears this out. I've lost count of how many times I've failed to get any kind of 3G signal with Orange inside a building when others on O2 or Vodafone are fine.
When the digital TV switchover is completed, the 800Mhz band will be opened up for the mobile operators which should improve the situation, but the 'other' networks are currently arguing that they deserve more spectrum allocation to redress the balance with Vodafone and O2, whilst Voda/O2 are saying any favouritism would amount to state aid which they'd challenge legally. Could get messy.