I got this one: http://www.dealextreme.com/details.dx/sku.6214
and never experienced any corruption with it for half a year now...
I have this one too. Although i have barely used it, it has worked fine and is a masterpiece! (apart from the colour lol).
The theoretical maximum size of a GC memory card is 128MBit, or 16Mbytes.
That card has 2x 128Mbit chips in it, with a switch, so you effectively have double the maximum capacity (or 2 cards in one slot lol). This also should theoretically allow easy isolation of savegame regions, which I've heard is a problem when mixing savegames (the card may format).
256Mbit is like a gazillion blocks, great for the GC backup launcher if u have heaps of backups