Just to clarify ...
If you need it for the extreme temp ranges that's one thing, but if you just need good performance, anything more than the Ultra II is a waste of money, the camera isn't fast enough to take advantage of faster cards.
You say "... the
camera isn't fast enough ..."
Do you mean the Canon 30D
isn't fast enough to take advantage of the Sandisk Extreme III cards?
I acknowledge Killer Angel's query mentioned that specific camera, but other camera's will certainly be able to take advantage of the extra "III" speed. My Nikon D200 gains a whole 1 second faster write-to-CF-card per image with the Sandisk Extreme III 4GB compared to the earlier Sandisk Ultra II 4GB card.
And I really appreciate that extra 1 Second when I choose to chimp ...
P.S. My timings are my own in-camera + stop-watch tests and not Rob Galbraith's figures.
Full RAW(uncompressed NEF) + high quality JPEG:
Sandisk Extreme III 4GB, 28 image burst - 8.3MB/Sec to CF card.
Sandisk Ultra II 4GB, 27 image burst - 5.3MB/Sec to CF card.
Edited to add that with both cards the D200 accepted 18 images on the fly, at 5FPS, before starting to queue the remainder. The balance of the 27(or 28) images were taken as fast as the D200 could process those earlier in its queue - but I'm in danger of digressing from the original question ... sorry.
Yes, it's definitely worth buying/using faster cards when possible