I've only tested the Focus 40 Blue braille display from Freedom Scientific on an iPhone 4. Almost all on-screen gestures have key combinations to execute the same commands on the braille display. Though the display's keys are rather noisy, the display is very responsive. I find that I have to retype braille more often than I have had to with other displays. However, the display is light and compact. The display is comfortable to use, and the braille is crisp. I would advise purchasing an Executive Products case for the Focus 40 Blue, though, as the standard case that ships with the unit does not allow one to operate the display when one is using the shoulder strap.
Texting is vastly easier with the Focus 40 Blue braille display than it is when using only the on-screen keyboard. Answering email messages is quite simple. I find composing email messages and using the calendar to be more difficult with the display than with the touch screen itself.
I wouldn't characterize the combination of a bluetooth braille display with an iPhone as as simple to use as a notetaker, but the price tag of the combination, as compared to a notetaker, makes me a happy customer. Besides, the braille display can also be paired with one's computer as well. And, Freedom Scientific's customer support has considerably improved since the early 2000's in terms of friendliness.