For whatever reason, the wider aspect (I don't have wide-aspect displays) and gently sloping edges feel more natural than the Intous3. The tablet does seem more responsive to lighter strokes, though only when directly connected to the PC. Through my hub, at least, there's always a slight delay sensing the stroke if it starts off with little pressure, and it doesn't register. I didn't have that problem when connected to a motherboard USB port. (Since I don't spend all day in Painter, I can't tell whether the response curve feels any different, though.)
For obvious reasons, the LED labels next to the ExpressKeys make them truly productive. Though it may seem an awfully mainstream application, I think a lot of users will find the media controls in the Radial Menu convenient; if not, you can reprogram them to your taste. While I'm not a big fan of using the RM for operations like cutting and pasting--it's easier to use keystrokes with my left hand, for example, even if the keyboard is off to the side--I found the menus great for launching Photoshop Actions. It would be nice if Wacom added the ability to load saved settings so that either the company or third parties could offer some presets; configuration, especially for multiple applications, can be a bit tedious.
I also wish there were more buttons, or a shift-type toggle that would increase the number of functions you could assign per button, and that the buttons weren't all identical. Though they tilt slightly, they're closely packed, and some sort of bumps or indents might help to make you a bit more secure that you're pressing the right one without having to look down. The Touch Dial is great, as long as you adjust the sensitivity appropriately. The button that cycles you through the different Touch Dial functions has only a tiny LED dot to indicate which position you're in, and I couldn't help wishing that it had labels as well; pulling up the onscreen reference can get quite tedious. And the Radial Menu has one seriously annoying behavior; it pops up under your cursor, but each subsequent submenu crawls up the screen rather than appearing in the same spot as the previous (even though the previous menu disappears). I frequently ended up with a submenu disappearing partly off the upper right corner of my display.
All in all, however, the Intuos4 is a great improvement over the Intuos3, and probably worth the upgrade if you spend all day with pen in hand. As usual, more casual users will likely find the price a bit off-putting, especially in the case of the $200-plus small version, which lacks the useful labels. However, if you've ever toyed with the idea of adding a tablet to your array of input devices, whether for business-related ink note taking, drawing, or for digital photo retouching, this is the model to jump in with.