Few general questions

[OpenTK capabilities]
OpenTK only provides bindings to OpenGL and doesn't impose any limits to the size and complexity of your scenes. The only limits are a) the hardware of the target PC and b) the capabilities of your renderer.

Now, the hardware of an average PC varies wildly in performance and features:

• Intel GPUs are the worst of the bunch, lacking both in features and performance. If you wish to support those, you will either have to avoid important OpenGL features completely (FBOs, shaders) or code separate rendering paths.
• Nvidia and Ati GPUs sold in the last 7-8 years are capable of OpenGL 2.1 (including shaders). Those sold during the last 2-3 years also support OpenGL 3.0+. Newer GPUs are generally faster and more flexible: R300/NV30 support shaders. R500/NV40 add dynamic branching and floating point textures. R600/NV50 add geometry shaders, transform feedback and filtered fp-textures.

In general, the older the hardware you wish to support, the more limited you are from a visual quality standpoint. It all boils down to a classic dilemma (trilemma): support for old hardware, visual effects, implementation cost - pick any two. What is your target audience?

[Alternatives]
As far as I am aware, OpenTK is the only library that a) is cross-platform and b) supports all OpenGL versions (1.0-3.1). The closest alternative is the Tao Framework which offers inferior OpenGL bindings and WinForms support.

Advantages of OpenTK (personal opinion):

• cross-platform support for Windows.Forms
• clean bindings to OpenGL
• active development
• upcoming OpenGL ES support (people from Novell have expressed interest in Android and iPhone ports)
• upcoming OpenCL support

The last two points may or may not be of interest, but additional flexibility never hurts.

Disadvantages:

• Relatively weak math library (parts are being rewritten to match the XNA API and improve speed)
• There will be breaking changes between the current release and OpenTK 1.0. Most of those will affect parts of the library you are (probably) not interested in, however a few will affect the math library. Mitigating factors: upgrade paths are usually available; you are not forced to upgrade from the 0.9.8 release (important bugfixes will be backported).

Of course, these are personal opinions. Other people may like/dislike different parts of the library.

If you don't care for cross-platform support, you can also use SlimDX (DirectX 9, 10, 10.1) or XNA. That latter is higher-level, hence easier to use than either SlimDX or OpenTK, but I don't know if it can be used inside Windows.Forms applications.

Your final choice is to use a full-fledged graphics engine instead of plain bindings: Irrlicht3d and Axiom are two engines usable by .Net applications. Using a graphics engine is generally easier than plain OpenGL or DirectX, however I do not know if either engine supports Windows.Forms directly.

[Applications using OpenTK]
I am sorry, but I cannot help you here, as the only list of applications is woefully incomplete. Several posts in the forums indicate commercial use, but the names of the applications are sadly not disclosed.

Personally, I have used OpenTK successfully in my thesis project, which involved building a platform for brain-computer interfaces. The frontend is using OpenTK to drive a virtual environment and can scale all the way from laptop IGPs to one wall CAVEs with stereoscopic rendering. With the exception of the GLControl (WinForms integration), this project used the whole breadth of OpenTK APIs with no serious issues. The aforementioned disadvantages (and the focus of the current work on OpenTK) is mainly based on observations from this project.

Objarni, i'm glad to see someone using OpenTK for such a complex project, if you're going to make a professional full-fledged CAD program. I hope that OpenTK is good enough for a large CAD/CAM solution.

TheFiddler, that answered most of my questions. Thank you.
Brain-computer interfacing... that's one very interesting subject.

I was thinking about using XNA some time ago, but i can't recall why did i eliminate XNA as an option... i think it had something to do with it's license, but i really can't recall why did i abandon that option. XNA would maybe be a perfect option for my project (it's windows only). SlimDX sounds interesting, but i have heard that it's far from complete, and can't be used in a professional application without heavy modifications and additions.

I was referring to the drawing space, modeling and rendering; i'm sure that GUI was done with WinForms or WPF.

SlimDX sounds interesting, but i have heard that it's far from complete, and can't be used in a professional application without heavy modifications and additions.
SlimDX is very complete (not 100%), but I doubt you will find any major feature missing. And if you are missing something, developers usuallly respond and implement missing features pretty fast.
And yes, you can use SlimDX on Windows.Forms. Actually all samples that comes with SlimDX usees Windows.Forms.

The reason why I had abandoned XNA was that it required both the .Net Framework and the XNA Framework installed in order to run applications. Although people had appropriate DirectX drivers, some of my testers had issues with the frameworks and the game would crash badly. Others would simply refuse to install the beta XNA Framework, knowing that "beta" (translated from "Microsoft language" to real language) means "alpha". Mind this was XNA 1.0, maybe the situation has changed nowadays. Just a friendly warning, because according to your description of the 3D capabilities you need, you would probably be better off with OpenGL (it would even run with unextended OpenGL 1.2, without any hardware accelerator present).