I may be very, very late with getting this info but I've been known to be late with almost every thing on a personal & planetary level:)

It seems that UDK can be used freely for developing games & that there are only two limitations: you must pay 99$ when you want to turn your game into commercial product & if your income hits 50.000$ you pay 25% of anything over that amount.

http://www.udk.com
http://www.udk.com/licensing

So I'm interested to know if any GS user has worked with UDK & what kind of experience was it? Is it easy/hard/extra hard working with this tools, how more complicated/easier is it compared to GS tools?

I made a map in UDK with my own textures and models for testing (as well as in other engines):
http://www.youtube.com/watch?v=dQqkkeRBAb0

Actually it is a great technology. The interface differs from others so it needs some getting used to. In the end it is very powerful and the latest version even comes with DX11 features.

Yeah, this looks really good, there is no mistake that the powerful engine is being used together with great 3d assets.

I must admit that my biggest fear is the programing part, how much custom programing did you have to do for this scene? Are there any good templates available to speed things up? Something like FPS player template, enemy AI template or maybe even shooter/rpg/platform templates?

Unreal Tournament 3 demo comes as the base template.

UDK is really different. There is visual script editor Kismet, which you can do alot of stuff with, and then there is unreal script for custom programming.

F.I. you may not need to write any unreal script for a simple platformer.

Wow that's great news. So would you say that Kismet is difficult to use? Can more-less standard projects be developed with Kismet or do you think learning unreal scripting is necessary?

This depends on the definition of more-less. Basic movement, collision, camera, triggers(events/cutscene) won't need much unreal-script.

Just asking: To which Script Language you would compare unreal script?

Sytax is java.

It's pretty much java with some extensions. And because of that extension, programming style is very different.

I wish it had LITE-C for programming needs