I've used RenderMonkey in the past, and while it certainly has its uses, ultimately it left me dissapointed. For a straightforward shader it's fine, but when you start getting into more complicated techniques it starts to break down. After clicking on a billion buttons to get my render targets and passes set up the way I needed I really wished I could just write some code. The lack of any ability to compute anything on the CPU side is what really frustrated me. What I ended up doing was computing a lot of things shader-side that in a real application would be done on the CPU, and it needlessly complicated the shaders.
Another avenue I've pursued is using the DirectX sample framework or an OpenGL sample, and working from there. Even in these simplified environments I find you end up doing a lot more bookkeeping then actual code. Additionally, OpenGL seems very unstable on my laptop -- even vanilla samples are crashing in the Nvidia DLLs.
The last couple days I've been playing around with Microsoft's XNA Studio 3.0 as a graphics prototyping tool, and my impression so far is very favorable. The API is pretty straightforward, and for the most part the abstractions seem in the right place. Porting the current thing I'm working on from C++ to C# took no time at all, and so far I've spent much more time writing meaningful code rather than working on scaffolding.
The GameComponent architecture they have is interesting -- for example, I found myself needing an orbit camera. Rather than write one myself, I just grabbed a component someone else has written. It was one of those few times where the code just dropped in. The only drawback is they haven't implemented taking input from the 360 controller, but that's easy enough for me to write and a lot less involved than doing the whole thing.
I was also surprised how easy it was to get my little project up and running on my Xbox 360. I didn't have to do any code changes, and it all Just Works. The debugging is solid but experienced Xbox 360 developers will miss all the nice tools you get with the real SDK. I'd be really nice if Microsoft released a lightweight version of PIX for XNA that worked with the 360, but I guess you can't have everything.
There have been some minor annoyances. Some of the C# syntax for dealing with vertex arrays can be cumbersome -- new'ing a Vector3 never feels right to me. You also lose access to some hardware features. For example, for some reason it thinks I can't create a floating point depth buffer on my laptop, when I'm very certain the GPU I have can handle that.
On the 360 side, they simplify a lot of the hardware details, but this has limitations. I can't find any way to resolve the depth buffer on the 360 to a texture in XNA. While these limitations are understandable given the intended audience of XNA, it is somewhat annoying.
All in all, I think it is a pretty good framework so far. I don't think you are going to max out the hardware with it, but for a large category of games it will work really well.