I just replaced my HID bulbs on my 99 GS400 with OEM HID's. I bought the PIAA D2S 6600K bulbs because the place I bought the bulbs did not have D2R's, and it seems to me by looking at the two bulbs side by side that the D2S would be better because they lack the reflective coating on the top of the bulb.
Anyway, I had to cut a notch in the plastic on the base of the bulb so they would fit in my housing, but this was very easy. The light looks great, it is slightly bluer than OEM, but it is not at all purple or ricey. One concern I had about the lack of the coating on the bulb was glare for oncoming drivers but I drove all over tonight without anyone flashing their brights at me.
All in all i think the PIAA bulbs are well worth it since I paid $300 for the pair, and the dealership wanted $370 for one, especially since there is a big improvement in light quality.