A high-speed memory card is only as fast as the camera you put it in.
Some camera processors can work with a high-speed card to decrease shot-to-shot time, extend video capture, and increase the number of shots you can take in a series in continuous-shooting mode. High-speed cards won't help you with shutter lag, though, since that has nothing to do with transferring photos to memory.
There are some cameras that advertise a level of performance that you can't attain with a non-high-speed card. For that reason, you should check the memory-card requirements on any camera that touts unlimited continuous shooting or enhanced video capture. Those are nice features to have, as long as the need to buy a high-speed card in addition to the camera doesn't come as an unpleasant postpurchase surprise.
Making sense of the numbers and labels on the plethora of memory cards currently available can seem tantamount to deciphering hieroglyphs. For help with this task, you can turn to CNET's "Digital camera memory-card selector
," our very own camera media Rosetta Stone.