I have a 4 Gig Hitachi microdrive. I REALLY LOVE IT. Its got so much storage for the money. Some people complain that its slow, its really not slow at all. Its still faster than a 40x Compact Flash Card, and benchmark test prove it.
The ONLY time i think your going to notice a difference in speed, is if your a hardcore continious shooter. My 4 Gig microdrive is always inside my camera for events like weddings. I am confident in its quality and performance.
I also own two 1 gig SanDisk Ultra II Compact Flash Cards, so my opinion is not 1 sided. I use my High Speed San Disk cards for when i shoot continious... sports, action, things of that nature... Or when i dont need mass storage.