I started in 2005 with the Ultra II series and have continually sold & upgraded as my needs dictated. I'm now using a Canon 5D Mark II which has HD video (I think we all know that by now), but the camera also creates a 30mb RAW file every time I snap the shutter. So, write speed is very important. I WAS using the Extreme IV series cards (45mb/sec), and even with the slower 4fps in the 5D2, the camera would hang after a few shots to write the data. Now with these new 16gb Extreme cards I get to hold down the shutter button and record WAY more images before the buffer starts to hold things up. What an improvement! This new series is worth every dollar. Remember, faster cards also give you longer battery life since the data writes faster, so that's another plus to these new Extreme cards. They are UDMA level 5 which is nice (the 5D2 handles up to 6). BTW, UDMA simply means that the card does a lot of the file processing, which gets you faster write speed. Non-UDMA cards make the camera do all the work when writing images to the card. If you have a UDMA-enabled camera, by all means get UDMA-enabled cards.

One last thing to help clear confusion on the card naming format: the 133X and 300X and all that simply means the speed that the card can write data. SanDisk doesn't use that prominently in their marketing, they tend to say "30mb/sec" or "60mb/sec", like that. Lexar uses the ###X format all the time. So when shopping around, keep this in mind:

SanDisk Ultra II: 15mb/sec (the original version) - Lexar calls it 100X (this older model SanDisk is NON-UDMA)
SanDisk Ultra II: 20mb/sec (the updated version) - Lexar calls it 133X (this older model SanDisk is NON-UDMA)
(Thanks Uri for the correction in Comments!)
SanDisk Extreme III: 20mb/sec (the original version) - Lexar calls it 133X (this older model SanDisk is NON-UDMA)
SanDisk Extreme III: 30mb/sec (the updated version) - Lexar calls it 200X (this discontinued model SanDisk is UDMA enabled)
(Thanks David for the correction in Comments!)
SanDisk Extreme IV: 45mb/sec - Lexar calls it 300X (The discontinued SanDisk Ducati line is also 45mb/sec and UDMA enabled)
SanDisk's New Extreme: 60mb/sec - Lexar calls it 400X
SanDisk's New Extreme Pro: TRUE 90mb/sec Read & Write - Lexar calls it 600X

Hope that helps understand all of this!

UPDATE (12-29-2009): I also wanted to mention that I've owned nearly 15 SanDisk CF cards since 2005 and I have never had ONE fail on me. I learned a trick from a pro: after you COPY (not MOVE) your images onto your PC, always format the card IN-CAMERA before you shoot again. Don't use Windows to delete your images off the card. The CF cards get grumpy for some reason (no matter what brand you use). I've shot 20,000 images on four different camera bodies, and never one card failure in four years.

UPDATE (3-30-2011): My two 16GB cards sre still my workhorse memory, use them almost daily, not one failure. This 60mb/sec line of cards is probably the best value for your buck, you get speed, reliability and storage room for not a lot of money (ha, you'd think I work for SanDisk, lol! Just a happy customer.)

UPDATE (7-12-2011): I did it... I splurged for a red/gold SanDisk 32GB Extreme Pro CF memory card - UDMA 90MB/s 600x (SDCFXP-032G-A91, US Retail Package). Wow, the write and copy speeds are even faster, amazing. It's been my main card these past couple months. But honestly, the red/black Extreme cards that are reviewed here are fast enough (even for video) and MUCH more affordable. As always, SanDisk cards are as stable as ever. Again, just make sure you COPY from the card to your computer, never delete images off the card using your PC. Then, format the card IN-CAMERA before each new use, and your cards will work and last a very long time. I know, I already wrote this in my first update, I can't stress it enough!

UPDATE (8/7/2012): Another year and 10,000+ more images shot including a wedding two weeks ago, and the SanDisk CFs keep running at 100%, ZERO failures. They do get stress tested between photos and video on my 5D2, and sometimes it get so hot here in Phoenix that the camera actually hangs due to overheating, yet the cards don't fail. Even if one did at this point it's irrelevant, I'm a SanDisk user for the long haul. In fact, I now have my editing PC running on a SanDisk Extreme SSD 120 GB SATA 6.0 Gb-s2.5-Inch Solid State Drive SDSSDX-120G-G25. Sorry if this sounds like a commercial, I REALLY don't work for them, lol! I just appreciate quality stuff.

UPDATE, 7/5/2013: Only the new Lexar 1000x cards are faster than the SanDisk Extreme Pro! The new Lexar 800x series cards can READ fast, but the write speeds are WAY slower than the SanDisk Extreme. That's why they are half the price of the 1000x. The 1000X are really fast, I use a few in my 5D Mark III and coupled with UDMA 7, we're talking 29 RAW images at 6 fps before the buffer kicks in. BUT, you will NOT see that kind of speed in earlier models that aren't UDMA 7. Therefore, The SanDisks are still the fastest in slightly older cameras like the 5D Mark II. Also, 5D Mark III owners, get this: The Lexar 1000X do scream at 140mb/sec in the 5D3, but only the 32GB and 64GB cards! The 16GB are 95mb/sec which is the same as the SanDisk. Weird, huh? The Lexar line is quirky, you have to know what you're getting. SanDisk is tried and true, and when they say 90mb/sec, they mean it, for both read and write speeds. Side Note: their Extreme II SSD in my PC is REALLY fast. ;-)

Okay, so when I recently purchased a new Camera (a Canon 7D, 18 mega-pixel stills + 1080p video camera) and I was convinced I needed the SanDisk Extreme Pro 90mb/s card to take full advantage of the 8 pictures per second burst rate this camera had to offer... then... I started looking at the prices for those cards. *wince*

So, after I'd picked my jaw off the floor, I came across this card. It's still UDMA (where the card does some of the processing, not just the camera, resulting in better speeds), still faster than any of the older Extreme (and Ultra) series cards including the Extreme IV's by a noticeable margin, and my money buys me much more storage space, at significantly lower cost. But... would it be fast 'enough'?

Well, I was still hesitant... but at almost 1/3 the cost (and since having no memory card makes my camera a little useless), I decided to pull the trigger. I dropped it in, fired it up, and put it to the test. I was expecting shutter lag... or the dreaded "busy" light flashing at me when I really let it fly on full speed. Much to my pleasant surprise, this never happened! I was able to rip away at full speed without any issues at all.

If you're thinking about buying a memory card for this camera (or another high-megapixel DSLR) and you want as much memory AND speed as your dollars can afford, with out sacrificing camera performance... this series of card is for you. I, personally, see no reason to require the MUCH more expensive Extreme Pro for use in this camera... well... unless you have money just burning a hole in your pocket. :)

I had been using SanDisk Extreme III (30 MB/s) in my Nikon digital SLR, and while they were reliable and so on, I was never able to get the full high speed frame rate Nikon promises. Instead of the 5-7 frames per second, I was getting more like 2-3.

Switching to the 60MB/s fixed that problem, and now I'm getting the full performance my camera is capable of...at least 5 fps under most conditions.

Interestingly, I also purchased a smaller card capable of 90MB/s, and saw no real difference, so I'm considering the 60MB/s version ideal - nice, since it's much less expensive than the faster cards.

Recommended if you need the best frame rate out of your camera.

I have had this 16-GB UDMA card for several years, using it with a Canon 7d that I bought when that camera first came out. I had forgotten I bought this card on Amazon, but in looking through my reviews tonight was reminded how much I have taken this card for granted. I shoot in RAW, so the files for each 18 megapixel picture are quite large. The 16 GB size holds all my pictures, as well as HD videos, some of which are over one GB in size. This card is fast and reliable, what I have always expected from SanDisk, but have only belatedly appreciated. Yes, I know there are faster cards out there now, but this one continues to serve me well!