Okay, so in that case I think what may need some getting used to is unbiased, physically correct rendering.
Vray, Mental Ray etc. were basically “faking” stuff in the past. In reality there is no such things as specular, reflection, diffuse etc. as separate entities - it’s all reflection. In the olden times, those things were invented to make rendering faster and (in the case of specular) make invisible lights visible and stuff like that.
In reality, all lightsources are visible objects and have a certain size, which defines how smooth their shadows will be.
And all surfaces either reflect the incoming light or let it through, so you only have reflection and transmittance, but a smooth surface will show sharp reflections and a rough one will show diffuse reflections.
And this is the core of unbiased, physically correct rendering - it works much more like reality.
So I personally find it much easier in the end, but after using the old-style stuff for so long, I needed a while to re-adjust.
Thea is not a hardcore-unbiased renderer like some others, so you at least have a diffuse setting in the basic shader (which is a bit of an allround shader), but there is no specular, only reflectance, and your surface roughness decides if it’s sharp or soft.
The glossy shader on the other hand does not have diffuse and is specifically meant for metals and transparent material like glass and water.
SSS is for Subsurface Scattering (relatively expensive to render - for most things the basic shaders “Translucent” setting will do).
Then you have ThinFilm for things like windows - it’s basically a glass shader for thin sheets of glass (single polygon) where refraction does not play much of a role - and it’s much faster to render than real glass. It also allows you to do soap-bubble like effects including the colouration based on physical reality.
And finally there is a coating shader, that you can put on top of for instance a wood material done with the basic shader and it gives you that typical coated look.
You combine them in layers (like a coated SSS or metal or mixtures via textures) and can also have things emit light, use displacement, clipping (for things like the alpha-channel of tree-leaves for instance), things can contain a medium (think visible light rays, but can be used for semi transparency etc. too).
There are also other areas where the physical realness hits you: You can’t for instance change the brightness of the sun. What you do instead is increase or decrease the ISO value of your display settings - if you’re doing photography, it should be rather logical actually.
And lights should usually be objects that emit light, with the least amount of polygons possible (for speed) and not touching or too close to other objects (against fireflies and long rendertimes). But there are several tutorials available on the Thea forum that should get you up and running rather quickly.
What you get out of it is: stuff looks real right away.
I got tired to try getting a phong shader look like anything real in the olden Mental Ray
I use “Presto MC” for pretty much all my normal work (using GPU and CPUs) but Thea has several rendercores, some biased, some not. TR1 and TR2 are much slower, only use the CPU but can render crazy complex lightpaths, caustics, multilayer materials etc. - for some pictures I rather render overnight and have all those bells and whistles.
I actually do not use the biased modes.
Try the demo with some of your own projects and see how it works for you.
I use it on a 6core i7 @ 4.1 Ghz, a GTX 660 TI and a GTX 980 TI and it flies.
On my laptop with a GeForce 840M with 2 GB RAM it also works fine, although slower of course.
Hope this helps