I bought the Lexar 8 GB UDMA card to use in a Canon 5D Mark II, which writes very large (20MB) files when you shoot in RAW mode (which I do). The place where I see an advantage is in downloading these large files to my computer.
Mated to a UDMA card reader, the card transfers about 72 files a minute or a bit better than one per second. My old SanDisk Extreme III card in a non-UDMA-enabled reader takes about 3-4 times longer. And of course if you have a camera that writes large files, 8GB gives you some headroom when you're out taking large number of photos -- it can take about 240 RAW images on my camera.
There's also an advantage to a high speed card like this if you shoot continuously with this type of camera, since files will be written more quickly with a fast card. If you have a UDMA-enabled camera and shoot often in continuous mode, this may be a worthwhile purchase for you. If you have a need for lots of capacity with any camera and you have or buy a UDMA-enabled card reader, this card will speed up file transfer very significantly. In my experience Lexar makes very reliable cards that do not have compatibility issues with major brands of digital SLRs.