The mantle is very much out of character with the driftwood that does provide a nice rustic texture and color. I would replace the mantle with a squared off piece of wood. Because the fireplace is off-center, rearrange the mantle display in an asymmetrical way to balance the position of the fireplace. The firescreen could be updated with something that is not so rigid looking and the bench area could be replaced with a nice stone slab (or tiles), maybe a light colored limestone. Do not paint the driftwood the same color as the wall or the fireplace will start to look like a black hole. If you paint it choose a color that is a little darker than the surrounding wall. This room could use some color. While you are focusing on this one element in the room, it is also important to keep in mind how all the parts of the room will work together.

Replace the current mantle with something much more organic, such as a slab of live edge wood. It would need to be squared off at each end, as suggested above.
I agree that the fire screen is not an asset to the fireplace.
I agree that the mantel decor should be as symmetrical. Hang the lovely artwork somewhere besides above the fireplace, and use something organic as a focal point for the mantel - a very interesting sedimentary or volcanic rock, a large burl, a collection of bird feathers...

You are right - it's an eye sore. Replace it.

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It looks to me in the pic there is no grout in the joints or not enough, I would grout the joints in a lighter warmer colour picked from the stone, this would soften the look of the fireplace,If you look at the left pic that Design-Duet added, this fireplace has been grouted and the grout looks flush with the stone.
I personally wouldn't paint it, I think you just add a different problem , your paint may look dirty in time.
Im sure you will get some great idea's from everyone.

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But I really like the modern version of the fireplace. [houzz=]