Despite its being out of vogue at the moment, I'm still a big believer in the faster-is-better philosophy. That said, I also recognize that it's not always worth the extra cost for some people, and that not everyone can or will always take advantage of it. So that's why I always take announcements about new, high-performance media cards, like SanDisk's 90 megabyte-per-second Extreme Pro line (along with Transcend's similar 600X CF), with a grain of salt. And after running some casual tests on the new card, I've concluded that for now, even only a limited number of pros will see worthwhile advantages to the faster model given the price premium you'll pay.
The point of using a faster card is to free up bottlenecks that constrain performance. In a camera, those occur when shooting in burst mode, when the camera needs to quickly offload data from the buffer to the card. So a card's speedy usefulness is a function of buffer size, file size, controller (interface) speed, and card write speed. On the downloading end, the bottlenecks occur when copying files from the card to the hard disk, where it's determined by the interface speed (USB or FireWire), media read speed, and operating system overhead. A faster card only helps if the card speed is, or contributes to, the bottleneck. So, for instance, if you're shooting burst but the buffer is sufficiently large and the files are sufficiently small, then a fast controller and write speed on the card don't matter. Same goes if the controller technology bandwidth is sufficiently lower than the card's.
What does that mean in real life? Comparing the UDMA 6 90MB/sec Extreme Pro and the UDMA 5 60MB/sec Extreme shooting a burst of 20 raw+JPEG shots in the Nikon D300s--a combined file size of about 19MB--yielded identical burst performance: 2.9 frames per second (fps). There was a difference in the time it took the camera to be ready to shoot again after the burst completed: 13.1 seconds for the Pro versus 14.4 seconds for the Extreme, or a small savings of 1.3 seconds.
With the Canon EOS 5D Mark II, a significantly higher resolution camera with combined JPEG+raw file sizes of 26.3MB, a frame rate differential of 0.2fps did occur--1.8fps with the Extreme versus 2.0fps with the Pro. But that's on average, because the 5D Mark II's buffer only holds 8 shots (the Nikon D300s' holds 12), so the speed difference only shows up after that. It then took 5.9 seconds before the buffer was ready again, compared with 7.8 seconds with the Extreme card. Similarly, with the 15-megapixel 50D, performance diverged by about 0.2fps once you passed the 10-shot buffer limit.
So you really won't see much of a gain unless you shoot relatively long bursts of raw+JPEG files with very high resolution cameras. And keep in mind that it doesn't necessarily apply to all high-resolution cameras. For instance, Sony's A900 and A850 only support up to UDMA 5.
On the download end, the real problem is readers and operating systems. Even SanDisk admits that it's only been able to attain close to 90MB/second speeds with its new ExpressCard reader, shipping in October, and only under OS X. (SanDisk couldn't get me one in time to test its performance.) Even the company's FireWire Extreme reader, currently one of the fastest on the market, only provided a significant performance advantage--39MB/second versus 37MB/second--running at FireWire 800 under Snow Leopard. Oddly, using the reader with a FireWire 400 interface, the Pro card was actually a lot slower, both on the Mac and the PC; And the fastest performance I saw was the older Extreme card, with a rate of 40MB/second on Vista 64 running FW400 (I don't have a FW800 interface for that system). I'm guessing there are some operating system optimization issues here.
The thing is, most pro cameras today are optimized for whatever the highest speed cards available were at the time they were developed, which makes it pretty hard to eke substantial performance gains out of them now, and it'll take a new controller technology, such as USB 3.0, before we'll see significantly better performance for people who aren't working on a 17-inch MacBook Pro. (Perfect time for Apple to remove the ExpressCard slot from the smaller notebooks. Argh.)
With the new card, SanDisk also improves the error correction capabilities for improved reliability--it uses 42-bit ECC up from 10-bit--which is always a plus.
Finally, for many people the price premium may not be worth the small gains. The Extreme Pro cards cost $811.99 (64GB), $507.99 (32GB), and $304.00 (16GB) compared with $375.99 (32GB) and $223.99 (16GB) for the plain old 60MB/second Extreme, which has all the same durability technologies that well, make it extreme. Of course, for a few, every microsecond of performance counts and the time may be worth the money. And if you need that 64GB capacity, well, you're stuck, since there's no 64GB version of the cheaper card, at least from SanDisk. But give it 9 months: once the price comes down a bit, more people have cameras that can benefit from the speed, and we've got the computer interfaces to handle the faster downloads and then I think it'll be a great deal.