Even though OpenWRT has been working on the WD MyNET N600 and N750 for a while, the signal strength on this device has always been lacking. Felix Kaechele recently went through the bootloader code for these devices and noticed that the LNA for both radios was never turned on in the initialisation - in fact, it was actively turned off!
A few patches later, and we're now seeing much more stable performance from these two devices.
While these patches haven't make it into the OpenWRT trunk yet, I've built firmware binaries with these patches built in. These can be downloaded from: http://openwrt.crc.id.au/.
UPDATE 02/02/14: I've built up an automated system that updates and builds OpenWRT for these two devices daily. The first build I added support for these devices is r39211. New images will be generated daily for the N600 and N750 devices. The first build available with only upstream code (not these patches) is r39412. All are available from my site.