The fact that the source is available does not mean a single thing about how the product is going to run on your server.
After all, it's you who downloads the software directly from the creaters, and it's you who installs it.
Where is the difference between you buying a cd with software and installing it?
As the others have mentioned, open source is subject to a lot more critisisms than M$, because everybody can look at the PHP source and see what could be the problem. All you can say about M$ products is that they are full of bugs and security holes. You can't look at the source and tell the M$ boys where the problem is. Which means they have to work from bug-reports, and issue a service pack (when they think it's important enough), so you can spend a few hours downloading a xx Meg file that fixes one bug and creates many new ones :-)
This is turning into an anti-M$ post... lovely :-)