Delivering superb performance, the Extreme IV flash card is far superior to the previous series cards in virtually every parameter. But you should be aware that the card can only give you its best when you insert it into an appropriate reader. To connect it to you PC, you should best use the Extreme series reader with a FireWire interface. The USB version of the reader cannot provide the bandwidth necessary for the card. The practical write speed of the card doesn’t reach the declared 40MB/s, being only 30MB/s, yet it is anyway two times the speed of the previous generation card, Extreme III.
By the way, if you have an Extreme III or want to buy it and save a little money, you should consider the USB version of the Extreme reader because this card performs better in it than in the old ImageMate USB reader or in the far more expensive FireWire version of the Extreme reader. It looks like different flash cards have their own preferences when it comes to the specific reader.
The Ultra II series card looks poor against the mentioned two and cannot give you a speed higher than 10MB/s.