I spent a solid 2 years and at a pace of around 80 - 100 hours per week working in FPSCx10. This is a 3D engine which is...SADLY...discountinued and hasn't been supported for a long time in favor of their earlier product FPSCx9.
What happened there would be like if everyone here utterly rejected C2 and after a year or so Scirra decided they should just go back to working on Construct Classic. A lot of the problems had to do with the botch operating system known as Vista. Many of the long time users were not going to leave behind their XP machines and DirectX10 does not work in XP.
But anyway....I had my fun their and made a slew of mini-games in a relatively short period of time. Learned absolutely everything was to know about that engine.
Now, I am starting over with C2 and not able to put in the 80-100 weeks like I used to do. However, C2 is so easy to use it is ridiculous.
I've spent time in 3D Rad (and agree about what others have said - it is a cool 3D engine), in Torque 3D, Unreal, and Unity - so far as 3D goes.
I built my own 2D engine using Processing (Java IDE) but the limitations of applets killed that one before I got too far. But it was a great experience and I *HIGHLY* recommend Processing to anyone interested in learned a bit more programming. To this day, if I need to do something 'unusual" like tangible user interfaces or biosensors, I use processing because of the lower level control.
I've worked in Ethanon a 2.5D engine based around C++ syntax with a decent but underdeveloped world builder. It has a fantastic lighting and shader system. But it is developed solely by one person in his spare time and out of the goodness of his heart (i.e. it is free).
I installed GameMaker...but found C2 before ever even messing with it and have no desire to at this point.
That about brings me up to present date. There have been a few other engines here and there but not really worth mentioning beyond what I've listed above.
Cool thread, btw!