Actually it is a bit more expensive than staying with MS for the moment, but the main criteria were stability, security and removal of the dependency of one company only (MS). The move is expected to pay of in the long term (>10 years). Cities are long-term planners, or at least should be.

That is why Ballmer failed to convince them to stay with MS by offering better prices. The fundamental motivation was not the current prices, but strategic reasons.

13000 computers? Crap, can you imagine setting up that many copies of any OS? (I know, there're tools for doing that, still sounds like a big task though)

Do you end up mirroring a disk 13000 times only to discover that you forgot to put kcalc on, or that the company intranet in the bookmarks list had changed?

Any switchover of this kind will run into teething problems. We switched over most of our academic admin office (about 30 computers) to Linux/OpenOffice. Despite the support of two experienced Linux sysadmins and backing from On High, there was considerable grousing that still continued some 6 months later. File opening speeds, minor formatting things, print speeds - anything that might be imagined to be a little worse than the good old Win/Office system. I think it is mostly two factors: resistance to any kind of change, and the loss of freedom to mess around with the system. The slight but obvious user interface issues are a good added excuse.

On the flip side, though: No viruses. No files lost. No idiot using someone else's machine and wiping out data. Automatic remote backup. The sysadmins are happy! Unfortunately these things do not seem to figure in the tally of the staff, even when one of their colleagues who has yet to switch has had all her files scrambled by one of the latest viruses.

In short - it is hard to get people to change. But there are enormous savings, and not just financially.