First off, I'm not extremely familiar with using C# for GUI development
in the .NET framework, so I'm not sure about the comparisons between the
appropriate .NET libraries and Swing.  Nevertheless, here's my
impression of the advantages and disadvantages of Swing as a GUI
library, independent of its cross-platform nature.

First and foremost, all things considered Swing is a fairly good
implementation of MVC using interface-based design.  That makes it very
easy to manage complexity in putting together an involved GUI for an
involved data model.

For example, I at one point inherited a GUI for (among other things)
managing an XML document, written in VB.  The product was riddled with
bugs, because every time some fairly complicated thing happened to the
structure of the XML document, it would also have to be repeated for a
set of tree controls that displayed the document.  The two would
frequently get out of sync and cause problems.  After experiencing this
problem for a few months, I was able to very quickly put together a Java
implementation of this same API, write a model adapter from a DOM tree
with the Level 2 events option to a TreeModel, and reproduce the user
interface with none of those problems.  Better yet, I don't have to
worry about ever facing such a problem again.

(Again, I don't know if GUI development for .NET improves upon what was
possible in Visual Basic or not; I can only speak for what I know.)

I also consider the flexibility of the Swing architecture to be a key
advantage.  For example, simply that the various complex controls such
as tables, lists, trees, etc. all delegate their low-level display to
renderers makes a lot of GUI development tasks much easier than they are
when the components are all rather discrete and separate, as they are in
many conventional GUI frameworks.

Layout managers are a huge plus for any GUI development based on the
fundamentals of AWT, so that advantage carries over to Swing.  Though
this way of doing layouts is sometimes painful to those accustomed to
working in other ways, the ability to describe what you *mean* in a
layout instead of laying out and handling resizing on a pixel basis is
invaluable.

As far as disadvantages, the top of that list for me will always be that
documentation is lacking in using some of the more remote corners;
particularly in look and feels and in some sections of the rich text
components - JEditorPane and JTextPane.

Swing performance will always be controversial, but I also feel safe in
making this statement about a disadvantage: it's probably easier to
obtain very high performance from most other GUI frameworks than it is
to obtain very high performance from Swing.

That's my initial thoughts,

Well my experience was, that .NET lacks a lot of common GUI components
(for example no moveable toolbar). Searching for free stuff on the
web, revealed that those additional stuff can be bought from 3rd
parties. Swing has almost everything atm, and you still get a lot of
stuff for free out there.

There are no seperate Layout Managers avaible. Even tho each component
has a "built-in" border layout. But that is often not enough.

I wasn't coding C# that long, so i never had a look if there is
anything like a replaceable Look&Feel (except default windows & xp
look)

About performance issues: swing guis and .NET guis don't make any
difference to me, sometimes i found .NET was an idea slower. But maybe
that was because i have ported GridBagLayout to #C#, where most
dialogues in .NET are supposed to be positioned absolutely.