We installed it on our two 64 bit seats a three weeks ago and so far, it's been very stable and a number of issues we had on SP3.0 have been resolved.
Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK
SW2009x64 SP4.0 Intel Core i7 2.94Ghz, 12Gb Ram, NVIDIA Quadro FX3700 Driver: 6.14.11.8246
SW2009x64 SP4.0 Intel Core 2 Duo 3.17Ghz, 8Gb Ram NVIDIA Quadro FX3700 Driver: 6.14.11.8246