What are the advantages/disadvantages of using Jython/IronPython/Pyjamas?
You are talking about two different things.
First, Jython and IronPython are Python implementations that you can embed in a Java or C# application to provide scripting capability. This reduces the amount of code that you have to write overall. Or you can look at them as a way to glue together and leverage an existing collection of class libraries. However you look at it, these are good things and lots of people use them.
But Pyjamas is something else entirely. It complicates your web stack and makes it harder to pass projects on to other programmers. The main use case is if you have a shop of Python programmers and they need to provide a rich Internet application client side but cannot afford the time to learn Javascript. Not as broadly useful.
Also, my personal experience is that most Python programmers already know Javascript reasonably well from building web apps. It really is not much of a learning curve to just dive into Javascript. I've written JSCRIPT scripts on Windows as a batch file replacement and bits of Javascript with Jquery in numerous web pages. When I wanted to learn server-side Javascript for node.js, it really only took a couple of weeks to round out my knowledge of Javascript. In my opinion, Pyjamas should be avoided. Sooner, rather than later, Javascript engines will be supporting version 1.8 of the language which greatly narrows the gap between the languages other than the curly braces issue.
EDIT
Okay, rookie here so please bear with me.  What I'm trying to ask is the following:

Is it plausible for a Python-syntax fan to use one of these options while other team members "plain vanilla" version?  Is it a matter of personal preference, or would it require converting other people to using these technologies as well?
Is it possible to easily convert between, say, Jython and Java or Pyjamas and Javascript?
Also, in general, what advantages/disadvantages have people experienced from using these in the "real world"?

I think that states a little more clearly what I'm looking for. Input from anyone who uses these technologies in the industry would be very helpful.
Thanks in advance for your insights.