Correct a condition where an incorrect response to a SMART counter will cause the m4 drive to become unresponsive after 5184 hours of Power-on time. The drive will recover after a power cycle, however, this failure will repeat once per hour after reaching this point. The condition will allow the end user to successfully update firmware, and poses no risk to user or system data stored on the drive.
If you've been running an m4 and haven't experienced any problems, the 0309 firmware update is still "STRONGLY RECOMMENDED" by Crucial (their caps, not mine). There's little doubt that this particular bug will strike m4 drives running older firmware. At least there appears to be no risk of data loss associated with the bug.
The folks in the Crucial forums seem to be having no problems applying the update. Quick benchmarks run by a few posters suggest that the 0309 update has little effect on performance, too. Crucial does, however, note that those with m4 SSDs attached to SAS expanders should wait for a separate update designed for that sort of configuration.