Pros: Memory size, durability.
Cons: None.
The bottom line: I always recommend Lexar and I highly recommend the 4GB to anyone who shoots a lot.
Like many, I was a bit hesitant to get a compact flash card that went above 2GB. What if something goes wrong and you lose all the pictures? As a professional photographer, that is always a concern especially if I'm on assignment.

When the 4GB 300x card came out, I was about to leave for a two month trip so I got two of them to go with the others, as they were on sale.

They have been wonderful!!! It's so nice not to have to change a card as often, especially if I'm in the middle of a great series of shots. It writes quickly (I shoot in RAW format) and so far I have had no problems whatsoever.

Lexar is also a name I very much trust in regards to the cards. The compact flash cards are very durable and can survive almost any condition. I was shooting some of the fires in California a few months back and it was windy, dusty and smoke-filled and the cards had no problem, even when I had to change them in the midst of a mini sandstorm!

If you are shooting jpg and don't do a lot of shooting, the 4GB might be a bit more than what you need, but for those who shoot a lot and often, the 4GB is perfect.