I got both.
Corsair XMS6400 TwinX
Geil 6400 Ultra Low Latency
Much prefer the corsair, and always have done.
for DDR, I have 3200 3500 and 4400 in platinum Flavours, and also 3200 and 4400 in TwinX in various PCs... All are Corsair but recently I have had a go at OCZ and been happy for stock systems ( Gone for corsair for the clockability of them )
Recently since going with Conroe, I have the 6400C4 Corsair XMS DDR2 and its been great, and only a couple of weeks ago I went for the geil.
Corsair and Geil are all rated identically, 4-4-4-12, the corsair is happy as larry at 1.8v which is what the Mobo ( DS3 ) Defautls to, but the Geil needs a minimum of 1.9v ( Fixed apparently although I have not needed to clear the BIOS yet and find out if thats true ) and so with Geil, there is an issue of needing to adjust the BIOS with some other RAM first before you use the Geil... Going from the Corsair to the Geil was not an issue for me there.
However, one thing I have noticed...
With the Corsair, I only took it to 400FSB ( 2.8Ghz ) and I got a fairly reliable 18 Second, 1MB SuperPI.
The Geil gets me 23???
I have now been running the Geil up to 450FSB which sounds like a very sweet overclock, but I am only hitting 19 seconds on a 1MB SuperPI.
My thoughts on that are simple... Geil are overhyped and they are nowhere near as fast as corsair.
They are orange however, so they look great.
Now, on this note, I will say though, that I seem to be havbing a few issues with my board cos the system does not seem to be running quite right, however, thats what I am getting.