For some users, one major usability issue on the iPhone is the lack of a removable battery and memory card. I haven't had a need for more memory or to replace the battery on my two-year old device, but the proliferation of SD cards for storing and sharing photos -- even the newest Apple MacBook Pros have SD card slots -- makes the memory card omission a legitimate demerit for the iPhone's usability.
But the most divisive issue for usability is the keyboard. Many people hate touch keyboards, such as that pioneered in the iPhone. The BlackBerry Bold's QWERTY keyboard is much easier to use off the bat, though I became as proficient on the iPhone's touch keyboard as I am on the Bold's physical keyboard after several weeks.
The Motorola Droid's touch keyboard is similar to the iPhone's, lacking only the latter's multilingual capabilities. The HTC Droid Eris's QWERTY keyboard is easier to use than either the Motorola Droid's or the iPhone's, thanks to a cool hold-and-press feature to get fast access to common special characters such as numerals, $, and !. The Motorola Droid's physical keyboard is very hard to use, with low-travel keys and flat surfaces that make touch-typing difficult due to lack of tactile feedback. Using it slowed me down immensely, and I quickly abandoned it in favor of the much faster touch keyboard. The difference between the Motorola Droid's physical keyboard and the BlackBerry Bold's is night and day.
The Palm Pre's keyboard is similar to the BlackBerry Bold's, but the glossy keys and red-on-black print make it harder to use, especially when you're hunting for a symbol. The BlackBerry Bold's matte finish and clearer text avoid that problem. But the Storm 2's traditional multiple-characters-per-key BlackBerry keyboard is difficult to work with, as the device has to predict which letter you want on each key. There's no reason RIM couldn't use the Bold's keyboard on the Storm 2, and it should have, especially because its touch keyboard is unusable, due to how the screen must flex and rebound as you tap each character. That creates a delay for each and every character, so it's slow as molasses. (The original Storm's screen actually had to click for each character, and the flex action was meant to be less intrusive -- it isn't.)
The other major input advance from the iPhone was the use of a multitouch screen, which lets you do gestures involving more than one finger. The use of gestures opens up almost unlimited possibilities for what you can control via touch. Most of the other touchscreen competitors (the HTC Droid Eris, BlackBerry Storm 2, and Pre) have adopted the same approach -- with the glaring exception of the Motorola Droid, whose bundled apps don't support gestures. That ill-advised decision puts the Motorola Droid at a major usability disadvantage.
Finally, the physical devices vary in size, shape, and button arrangement, but by and large, they all have good designs. The Palm Pre feels the cheapest of the bunch in terms of finish and materials quality, and its fuzzy screen is an issue. The Motorola Droid auto-adjusts for brightness, using a light sensor to determine the screen intensity, but I found that it often flickered as it continually adjusted brightness in naturally lit rooms. Its on/off buttton is also hard to access.
Although these devices are attractive for potential business or professional use, they're also meant to be fun personal devices. The iPhone leans heavily on iTunes for access to music and videos -- after all, the iPhone is also an iPod. There's a reason the folks at Palm keep trying to hack iTunes to allow syncing to the Pre.
The other devices can all play music and videos, using Internet and carrier services for over-the-air access and USB syncing and SD cards for access to PC-based files. (Android users should get the DoubleTwist app for Macs and PCs that syncs Android devices with iTunes libraries.)
The Motorola Droid boasts a 5-megapixel camera with LED flash, giving it the most capable built-in camera based on the specs. But as my Chicago Sun-Times colleague Andy Ihnatko has shown, the iPhone 3G S's 3-megapixel camera takes better pictures most of the time, likely due to its software's correction capabilities. The Pre, BlackBerry Storm 2, and HTC Droid Eris have midquality 3-megapixel cameras, while the BlackBerry Bold has a low-quality 2-megapixel camera. All but the Bold's are capable of video capture as well.
The iPhone stands out for its gaming and entertainment capabilities. There are hundreds of cool games and playtime apps that can amuse you for hours. The other devices have a much more limited selection of playthings.