Keep in mind the known good cards may vary between devices too. The Samsung Focus I have seems to be very picky, and owners have discovered registry settings (yes, Microsoft even has a registry in their new mobile OS) that help. Mostly related to how the phone deals with power management of the SD card.

If the phone doesn't like the SD card, it may resume from sleep slow, run slower in general, or have stability issues. The really bad SD cards will also cause the phone to reset, and in certain cases, data loss as well.