I've got both the Sandisk Extreme III and a Hitachi 6Gb.
My plan is to use the Solid state card for my UW stuff because in the unlikely event of a flood, the microdrive would be more at risk.
I very rarely fill the cameras buffer (Canon 5D) so the speed of the card really isn't something I notice.
Only on 2 occasions have I noticed the wait... the first was taking some surfing pictures (and I can't remember which card I used) and the second was at the Professional Bull Riding competition last weekend. I was shooting with the microdrive and found the wait a little too long. I think the speed of the Sandisk would have been nice but I chose capacity over speed for that event.
As an aside, shooting at ISO800+ didn't turn out too badly. I had hoped to stick with 400 but when those cowbos go flyin' they are really moving!!