The FAQ states that Kingston's MMC+ cards should be avoided but I've had no problems (at least not directly caused by the card) with my 512MB card. I've had some stability problems but I don't think they are related to the memory card.
Other people not having any problems with MMC+ cards:
http://www.n-gage-help.com/modules.php?name=Forums&file=viewtopic&t=4857
Anttoni Jaakkola
