I use SD cards. I've used SD cards exclusively since I got my first Nikon point and shoot. I currently have three SD cards in my collection. One is a 2 GB which I have used previously in my Nikon D50. One is a SP Silicon Power 4 GB which my wife bought me since my Sandisk Extreme III 2GB cracked under the pressure of being the sole professional grade SD card that I owned. Now I recently purchased an 8GB Sandisk Extreme Pro (the brand that replaced the Sandisk Extreme III) 8 GB SDHC (my D300s can take SDHC cards).
My Current SD Cards (from smallest capacity to largest)
better photos to come when I get a 105mm VR Macro lens
My viewpoint on memory cards is that the more memory that you have in one card, the more likely it is that you will lose all your images if a memory card goes corrupt or heaven forbid, crack under pressure like my 2GB Sandisk Extreme III. Hence the reason why I shoot my wildlife images with 8GB. That way, if one of my cards goes corrupt, I will have only lost the average of about 382 images.
The way most professionals carry their memory cards around with them is with a memory card wallet. You can get various types ranging from tough nylon fabric to metal cases. I prefer to carry mine in an Optex "Secure Digital Case". Made from metal, it carries 6 SD cards though variants can be found for different type of cards from CF up to Sony Memory Sticks. These offer a mode of protection against impact, dirt and grime all of which can ruin a memory card very easily.
At some point in my career, I will do the professional thing and get enough SD cards to max out the carrying capacity of my Optex cardcase and will have to purchase another. I will probably attempt to go Pelican then. My hope is to have at least 6 8GB capacity SDHC cards in my cardcase. All Sandisk Extreme Pro cards. I like the reliability of the Sandisk brand. I started out with a Lexar when I had the E2200 point and shoot, however, I prefer Sandisk Extreme Pro over Lexar Pro cards.
My D300s has the capacity to take both SDHC (Secure Digital High Capacity) and CF (Compact Flash) cards so I could choose between either carrying SD cards or choosing to carry CF cards. I find that CF cards are a little more expensive by about $10.00. I'm cheap so I opted to go with the slightly more fragile, but still useful SDHC. Most people tend to go after CF cards. You can pretty much do anything to them short of running over them with a car and they tend to not be so fragile. But for me, I'm not that picky. I'll take the cost savings over the high-impact resistance of the CF card any day. Eventually, though I will probably cave in and get CF cards as I will be probably upgrading to the Nikon D4 or equivalent in the future. I have no use for Hi-Cap CF cards right now...other than to shoot video. Which I rarely do. I will eventually once I get my 600mm f/4 but until then, shooting bird videos for me with the Nikon AF-S 70-200mm f/2.8 VRII is just a waste of time.