I have done one real web using .NET, but I played with it a while to see if I liked it. It is good, very good in some areas, there are also some bugs in .NET3 that were present in ,NET1.x. But .NET with C# is incredibly easy, so I expect it will in time become popular mainly because students will find it easier to learn from scratch than PHP, Java or Perl.
Mono was going great guns. What they say is that Mono is not .NET, and that is true, but still its on the same lines and they looked destined to bump into one another at some stage.
The main problem with mono was the documentation. You would set off on a route to write a function, then come to call a mono lib, look it up on the mono web only to find yet another "This document has not ben written yet".
Now, once you are experienced in any language, in many cases could guess many parameters. Not all details of a function, but you don't always need the fine details.
All the mono writers had to do was give us the 'header' information for functions and constants and we could have worked it out.
But on something as new as mono with different libs to the norm, you can not guess.
Mono ought to be a major one by now, the simplicity of .NET but more easily multi-programming language, I would have loved mono-php. But without documentation it just isn't worth the hassle of downloading it.
Try for yourself, take a look at the documentation site below and browse a few functional areas. What you will find is that areas that programmers find 'exciting' are written, not always well but at least they have enough so you can use it. But areas that are important but dull from a programmers persective are not written.
So POSIX, Security, Maths, MPsql and many more either have huge gaps in the documentation or at best are skimpy, essential though these areas are, they are dull. But GTK is exciting, so that is done, over done probably.
So, much as I would love to sing the praises of mono, I simply can not do it in the state it is currently in.