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In my opinion...it's worthless if handholding as it kills a lot of the stabilization you get by having the camera up to your face. If you are propping the camera on something or on a tripod, it works fine...but in bright sunlight, you will have issues with seeing the back screen.
I don't have a D300s but have used Live View for macros on the D7000 and previously on a D90. I'd bet it's similar.
I'll echo some of the other comments and add my own. I would not use it it for hand-held shots -- only for tripod mounted shots. The angle of the screen may be too awkward (though putting your face to the viewfinder can be equally bad unless you use a right-angle finder for some shooting).
I avoid shooting macro in direct sunlight but still have to turn the LCD to its brightest and sometimes it's still difficult to see. In direct sunlight it'd be much worse and possibly unusable.
There is a slight time lag in the screen refresh though it's not too bad. It's better to turn the focusing ring slowly and work it back and forth as you get the final focus.
A large advantage is that I can zoom the live view in to 1:2 or even 1:1 (LCD goes all the way to 2:1). If I'm trying to get the pollen grains on the tip of an anther in focus, the live view definitely helps for that kind of shot.
I'm finding I use it more often than I don't, so it's a positive feature for my shooting.
On a tripod, its superb - astonishing how much more accurate the focussing is when you zoom in to 100%: however good I think my focussing skills are, live view ALWAYS exposes at least a small degree of imperfection, and enables me to correct it.