I think it's very close to too saturated but I tend to like vibrant and intense colors so I'm on board with this shot.
My first reaction was not that the sky's color is unnatural - some long exposures take that tint quite automatically. The only weird thing is that around your trees there are subtle halos of a lighter blue?
I very much enjoy the composition of this shot.
I like the composition but agree with the comments about oversaturation. Also, the lighting in general seems a bit harsh, but i'm not sure if it is from a slight overexposure or the hard sidelighting, maybe a combination of both. What time of day was this? The hard shadows in the foreground are somewhat distracting. I love what you're going for here though, punchy color and a good pleasing scene.
It's a typical tell-tale indicator that something like photomatix was used. In said program, it's very easy to take one slider and take saturation from 'hey neat' to 'oh my dear lord'.why does everyone assume something saturated is HDR?