I wouldn't discount the advice of others here, however-- this is my own experience.
I jumped on the Crucial bandwagon in summer, 2007, when some of our gurus here gave them a favorable eye, or at least when there were many enthusiasts who thought highly of them. In fact, Anandtech published an article -- probably within the previous 12-month period -- showing the flexibility of Ballistix modules for tight latencies and over-clocking. They may even have compared DDR2-667 with DDR2-800 choices.
Like others who will tell you, I set the voltage on a pair of DDR2-1000 Ballistix to just below the warranty maximum. And eventually, they went south after a few months. I picked up two kits of rebadged DDR2-800 Tracers [Tracers and Ballistix are essentially the same but for the LED lights] -- getting them on sale. With those latter, I ran them at 0.075V below the warranty maximum and over-clocked them modestly within a range between 835 and 875 Mhz. Those, too, blinked out after exactly one year.
I recently replaced the remaining 2GB kit with a 4GB kit of G.SKILL DDR2-1000's. And that's another story with a mixed ending, because I can't get the latter to run at DDR=835 Mhz and minimum warranty voltage at latencies tighter than the spec.
It's my opinion that Crucial/Micron had spec'd their modules 0.1V higher than they should have as a matter of business and engineering prudence, or conversely -- they didn't test them adequately. You will notice that more recent issues of 4GB Crucial kits are now spec'd to run at a 2.0V maximum -- instead of 2.1V.
Now -- sorry about the length of this post -- let me end it with observations about RMA'ing with Crucial.
There is no guarantee that you will get modules as good as the ones with which you started. I only suspect this scenario. They had so many RMA's on these suckers that they began to test the returned modules -- binning the "good" ones to re-package in kits and send back as replacements in their RMA process.
With my Ballistix DDR2-1000 sticks, they returned a set which "met the spec" alright, but you couldn't run them at 1T command-rate as all the others would run new out of the box. When I contacted them, they said "the modules weren't spec'd to run at 1T" -- that doing so would void the warranty. That may be true. But why, when the original three kits would run at 1T (even though I heeded their advice and set them later to 2T) -- would a replacement kit of the same DDR2-1000 modules be unable to run initially at 1T under any reasonable settings or slightly higher voltage than they ran at 2T and the same latency settings?
My conclusion: I didn't get "brand-new" replacement modules when I RMA'd.
You decide whether it's worth your trouble . . . .