Here are my initial impressions on the iPhone (after the first day).
In one word, AWESOME!
I have used and evaluated practically every smartphone devices in the U.S. market for the past couple of years, ranging from the Treo, BlackBerry, various types of Windows Mobile devices, Symbian devices (Nokia, Sony Ericsson, etc.), and others. I have also worked with hundreds of MIT's faculty, staff, and students and recommended devices that fit their needs. But, this iPhone is something else entirely different and amazing. It actually puts a smile on my face.
I had many doubts before I tried it, like many of the skeptics. Lack of physical keyboard, absence of 3G data connectivity, etc. Although I've only spent a day with the iPhone, I am extremely impressed, and I believe that the iPhone will put smiles on millions of users.
First and foremost, I think it is important to recognize the vision of the iPhone. Whereas every competitor was busy copying one another (notice how every one tried to be just like BlackBerry with push email and qwerty keypad) and caving to the perceived and real demands of the all-powerful carriers (Palm's excuse for not having wi-fi in Treo's), the iPhone was able to defy the conventional thinking in true Apple fashion and show us something entirely different. It looks like this could be the beginning of a repeat of the iPod revolution.
Specifics:
Keyboard: it works as advertised, and I do not miss the physical keyboard on Treo, BlackBerry or other copycats. Users need to think different and not attempt two thumb-method, as the iPhone's form factor is not designed for that. But, if one learns to use and trust the iPhone's keyboard and associated software intelligence, it is more than sufficient for writing short emails or even a long memo.
Lack of 3G: While it is true that no marketing hype can make a slow connection any faster, for what iPhone is designed to do, the EDGE network seems sufficient at this time. In fact, this is also true of the BlackBerry devices. Both iPhone and BlackBerry devices really function adequately with 2.5G (EDGE and 1xRTT on CDMA networks) while maximizing the battery life. As a case in point, the battery life of a BlackBerry with EDGE is much longer than the one with 3G connectivity without sacrificing the usability for the most part because the BlackBerry's push-email was originally designed for slower network speed with short bursts of push-email data flowing in the background (with the exception of large file downloads and using the device as a modem for a laptop). When better technology allows the iPhone to gain
data speed without sacrificing the battery life too much, it will only get better.
Screen: the best I've seen! This is one of the best part about the iPhone. Not only is the screen larger physically, but it has much higher resolution thanks to iPhone's focus on multimedia. It works great both indoors and outdoors.
Mult-touch interface: definitely a major coup. Works because of the larger screen. Another thing that demonstrates the iPhone's difference in philosophy.
Real Web Browing!
This it not a first (Nokia had similar web browser built in with some of the E Series devices from more than a year ago), but coupled with large display and multi-touch, iPhone's implementation is so much better and useable! While the EDGE network degrades the experience somewhat, it is a joy to use! I am confident that Apple will only improve on this with the addition of Flash support, security certificate support (important for enterprise users), and a few other minor things in the near future.
Real iPod in a smartphone! Although I have dozens of smartphone at my disposal that can play music and movies, I have never bothered. But, iPhone just makes you want to put on those white headphones and enjoy the music. (I am writing this email on the iPhone while enjoying my favorite ABBA tracks.)
The iPhone is not perfect yet, and it might not replace the Treo's and the BlackBerry's today. (I already have a list of items that are essential or of interest to MIT users, e.g. Wireless synchronization of calendar data, VOIP, personal certificate support, etc.) But, it is a great start for Apple and I am confident that it will soon be even better.
Life is good with an iPhone! 
(Drafted using the iPhone)