I have now had 4 sandisk SD cards (128-no longer have, 256, 2 512s) and one lexar (256-no longer have), and I have yet to experience problems with the sandisk cards (or the lexar for that matter). I have used the cards for dual booting on my 5500, have written and re-written many times, and have not had problems. With the latest mmcsd.o module and the asyncio kernel provided by Derekp, I'm getting some very good (for my use) speeds in reading/writing. Using home-on-sd, I can rarely notice that I'm working off of SD (again, depending on what I'm doing).
From memory, there was a potential bad batch of sandisk sd cards about 1 year ago or more, leading many to doubt them. However, here's a review from pdabuyersguide.com:
"SanDisk SD (Secure Digital) Cards
www.sandisk.com, prices vary with capacity, 5 year warranty
SanDisk is one of the big names in the storage card arena. Heck, they invented the CompactFlash card in 1994, and worked jointly with Matsushita (known mostly by the name Panasonic in this country) and Toshiba to develop the SD memory card. We've been using their CF cards for years and the SD cards have worked perfectly for us in a Palm m130, i705, Casio E-200 and iPAQ 3835. We've tested their 32 meg and 128 meg cards. Transfer speeds are excellent, and we haven't experienced MediaPlayer skips and stutters on the iPAQ 3800 series (that iPAQ has some speed issues with SD cards because of the way Compaq implemented SD). SanDisk is the OEM manufacturer of Compaq SD cards. Buying hint: Compaq marks up the price of their cards, buy SanDisk instead! The cards come with a protective case. "
You will find faster cards in the market more and more (AFAIK, the question remains what speed improvements each zaurus model can take advantage of), but I would seriously doubt if the cards are flawed based on the time and effort that sandisk has spent in the market and my personal experience.