OWC is the only 3rd party provider I know of that sells Apple original factory Samsung DIMMS.
Generally Samsung and Micron (Crucial) have all the correct specs including
the correct SPD specifications (Serial Presence Detect) Chip
What makes RAM " Apple Compatible" Thank you CanadaRAM
1) the arrangement of rows and columns of the RAM banks and the chip density -- for example many Macs can't recognize high density chips, so an otherwise identical module with 8 high-density chips on it mightn't work, where one with 16 low-density chips would. Generally it is cheaper to make modules with fewer chips, so the cheapest modules tend not to work in certain Macs.
2) the settings burned into the Serial Presence Detect Chip or SPD. Apple is much stricter about SPD values than PC makers. But you never get to compare what the SPD settings are because these are never advertised. That's why you need to rely on the seller to test the RAM and guarantee compatibility, or you can take your chances with generic RAM.
From the Developer Notes: "Important: For a DIMM to be recognized by the startup software, the SPD feature must be programmed properly to indicate the timing modes supported by the DIMM."
What you can get away with on one Mac doesn't necessarily mean it will work in another. PowerMac G5's pre Oct 19 were relatively tolerant, where iMac G5's and Minis are much fussier for compatibility on the nominally identical modules. Powerbook G4 1.5 GHz machines will take modules that the 1.0, 1.25 and 1.33 GHz 'books choke on.
The problem with marginally compatible modules is that Apple can also change the compatibility picture with a Firmware update or a new OSX. Lots of RAM that just slipped under the wire and worked with OSX 10.3 failed when the machines were upgraded to OSX 10.4, just because Apple tightened the tests for adhering to standard.
Latency is mostly a non-issue with Macs. Apple states that their machines run with a range of latency values... and very slow latency RAM will slow the machine down. In order to exploit lower latency, however, the motherboard must be able to change its timings, and there is no evidence from Apple that any Mac can take advantage of latencies faster than CL3 (CL4 on DDR-2).
(it's the same in the PC world, the motherboard has to either have Intel's "PAT" or equivalent automatic latency adjustment built in, or you have to manually change it in the BIOS setup)