I have 2x2GB Crucial Ballistix and 2x2GB Mushkin Redline. The Ballistix is actually DDR2-800 rated for 4-4-4-12 @ 2.0V, though it also has an EPP with 5-5-5-15 timings at 1066MHz, 2.0V. The Mushkin is DDR2-1066 rated for 5-5-5-12 at 2.0-2.1V.
They run fine on JEDEC specs (DDR2-800 at 5-5-5-15, 1.8V) and on overclocked settings (DDR2-1066 5-5-5-15 at 2.0V).
I can also run them at 800MHz, 4-4-4-12, 2.0V. The way the two sets seem to match so well in performance makes me think they use similar memory chips....
My little brother has two sets of ram that are certainly not the same. 2x1GB Crucial Ballistix (the 2.2V ones) and 2x1GB Geil. I say different because the Ballistix can OC to 1000MHz+ but the Geil set can't OC much from 800MHz. Both are rated for 4-4-4-12 timings at 800MHz though. Those worked well together at JEDEC specs.
My old comps also ran on mixed brands (value ram at matched speeds). Those worked alright as well.
TL;DR I've never had issues with mixing ram brands.