Ok, so I've heard of it. AVISynth. What is it exactly? Some sort of open source post production FX editor?

Go to the AviSynth website:

http://www.avisynth.org/
The FAQ: http://www.avisynth.org/AviSynthFAQ

Has anyone used this? Experiences?

Yes. It is like imagemagick for video. Very useful for batch operations. Not so useful for things that you need to finely tweak because there is no WYSIWYG interface.

Avisynth is a great tool that has a lot more power than what most people think it does. Not only can you filter and convert video but you can also edit together multiple segments and animate layers. Think of it as a comand line version of a fully working NLE. The problem is that it is a comand line only text only NLE. This makes it kind of hard to use.

Think of Avisynth as a codec, that accepts script commands, not compressed video.

So basically you create an Avisynth script using a text editor, with a bunch of commands indicating the video source, and the various transformations to apply.

Then you open it up in any AVI compatible application. The app will hand it off to the Avisynth "codec" which will return video frames back to the application. VirtualDub appears to be especially friendly to Avisynth scripts.

So you could basically write an Avisynth script that opens up a series of files, deinterlaces thems, resizes, blends etc. If you open the script in your NLE, it will "see" the results of the Avisynth commands as if they were just a regular video file. Really quite amazing.

Since Avisynth scripts are just textfiles, it is easy to write programs or scripts that automate repetitive operations.

As far as I can tell, Avisynth contains the best tools for advanced deinterlacing and resizing of video.