I have a Canon 7D, and use a number of CF cards, most of which are 'previous model' Sandisk Extreme III and an Ultra II - which are rated at 30MB/s. Note - some time back Sandisk changed the ratings of their disks, so when someone says "Sandisk Extreme n or Ultra n" it actually depends when that card was made (better to use the 'MB/s rating' if displayed).
Back in the 'olden days' (ie when I had my first digital P&S that used CF and my old HP PDA) I also had a few other types and brands (eg Sandisk standard 'blue', Kingston, Transcend, etc) But some of these are so small, that they only large enough to hold 1 or 2 photos from my cameras now!
Several years ago, I did a lot of testing of the speeds of those 'old' CF cards (mainly in my USB2.0 card reader), and actually there
was quite a lot of difference in both read/write speed. I did the same tests with USB1.1 and USB2.0 flash memory (the 'USB thumb drive' type). I won't bore you all with the details, but just share a few insights: a) the faster CF cards had similar write speeds to the fastest USB2.0 thumb drives (back about 5 years ago) b) the (Sandisk) Micro Cruzer drives I bought for myself and another for a friend were both consistently slow and seemed to be very 'fussy' about format (the exact 'block' size, etc c) I did buy a faulty USB drive (would often 'corrupt' data). I took it back and replacement was fine.
Now to answer the OP's question. My more recent experience is that the Sandisk cards that have 30MB/s disks work well for my normal shooting with the 7D (I frequently do a burst of 3 to 5 usually large JPEGS). Occasionally (eg ad-hoc sports or small fast birds in flight), I will take more - but nearly always just JPEG. However I have done tests using my 30MB/s and other (slower) cards and if shooting RAW (and esp RAW+JPEG) in a burst, it seems to slow down after 1.5 to just over 2 seconds of burst (when the camera is writing images from buffer memory to CF memory). I've used a faster card from a friend, (UDMA) - and the write to card is noticably quicker.
If I will get another card, size and reliability is probably more important to me than speed - and I'll probably get Sandisk again - because of their reputation and guarantee here in Australia. I would probably get a fast card (something 60MB/s or faster) 'just in case' I ever get into more sports or will change my workflow to be RAW. I've never had a hiccup with any Sandisk brand CF card (in terms of reliability, formatting, corruption).
Finally, on the 'aside' - I've found the Sandisk software that comes with the latest few CF cards I've bought great for recovering 'lost photos'. Actually on 2 separate occasions I have recovered data from friends' memory cards (one from a P&S and another a DSLR) who 'thought' they had lost all their precious photos (read: hundres family and holiday!) In one case I recovered about 80% of photos from a SD card where the camera (or user) had accidentally deleted the photos, and in the other case all but about 2 or 3 photos (several hundred restored). My friends were so thankful to me for recovering it. I always encourage people to back up photos (and save in another building, so that in case of theft or natural disastor - one set might be saved!) :
Regards
Paul
