[From Kresten McGrath] I am a long-time committed Treo user but even I could not resist the lure of the iPhone and I purchased one a few days after it was released. I used it heavily for over a week and then returned it. Here are some reasons why the iPhone, despite its beauty and elegance, can't beat the Treo for me when it comes to real-world usage:
1. My Sprint Treo 755p has better coverage. In most areas where I was, the iPhone had a good signal. But in the building where I work the signal is too weak for the phone to work reliably. We have no Wi-Fi where I work either so the phone /data capability of the iPhone became useless for 45-50 hours a week. Unacceptable. My Treo, on the other hand, has full 3-4 bars of service in my building and seems to hold a signal *everywhere* I go.
2. My Treo has a built-in tasklist application. I am a list addict and use my tasklist on my treo probably 20 times a day. I can pop my treo out of its holster, hit the task button and put a task down, and have my treo reholstered in about six seconds. This means I can keep those stray thoughts or to-dos I notice as I navigate my day fully captured and organized. The iPhone has no tasklist application.
3. My Treo has cut-and-paste capability. The iPhone doesn't. Yes, you cannot highlight any text on the iPhone and copy it anywhere else. 
4. My Treo has a memo application that actually syncs with my desktop. I use the memo application to enter all sorts of miscellaneous data, from registration codes for my various programs, to recipes, to business critical information I use on my job every day. Entries in my Treo's memo application are searchable in a matter of seconds. Each memo syncs with my computer, either to the palm desktop application or to Outlook. With the iPhone, there is a notes application but it doesn't sync with anything. And since there is no cut-and-paste, you cannot even cut information from a webpage or email and paste it into your notes. Pretty useless.
5. My Treo keeps track of categories in my contacts. With the iPhone, any category information you have for your contacts is useless. You cannot filter your contacts by category. Every person you have in your contacts is presented in one big list which you then have to flick through alphabetically. I saw someone in a store the other day that I knew but I couldn't remember his name. I went to my iPhone to pull up the category where I knew his entry was and couldn't--he was lost in the 3000 entries that make up my phonebook.
6. Battery life was dismal on the iPhone. I charged it each night and shortly after noon it was at low battery with minimal usage. My Treo will usually last all day. I also have a spare battery that I can pop in at any time.
7. Vibrate was too weak on the iPhone. I am in numerous meetings throughout my day and always keep my phone on vibrate. My Treo's vibrate is very strong and can get my attention. The iPhone has a vibrate feature but it was so underpowered that I was not able to feel it most of the time. I had to keep the sound on in order not to miss all of my calls. Then, with the default list of ringtones available on the iPhone, I could not find one that I felt was "business professional" enough to have play in the middle of a meeting (the dog-barking ringtone was fun and all but was I looking for more of a subtle chime tone).
8. The email implementation is poorly done on the iPhone. With my Treo, I can configure how much of any message I would like to download. I can also configure notification options for individual email accounts. With the iPhone, only the headers of your email are downloaded. When you click on a message to read it, the body of the message is then downloaded. When you are not in Wi-Fi coverage, this is slow. I could find no setting that would tell the iPhone to download the message first before alerting me that I have new email waiting. Also, on the iPhone, the number of emails your have unread is shown to you only in the main application page. You have no idea which accounts this unread email is sitting in, in your business-critical email account, or your not-so-critical personal accounts. Another frustration was having to tap into each account (Tap email->then account->then inbox->read messages, then back->back->other email account->inbox->read messages). With the Treo, my email alerts pop up to tell me which email account has pending message. If I click the alert, I am taken right to the waiting email in the correct account. I also have the ability to have a visual notification on the Treo when I have email waiting--I can look at my Treo sitting idle on my desk with the screen off and tell by the LED on top if I have email waiting. I won't even mention that my Treo can wirelessly connect to my work's exchange server for true push email while the iPhone cannot (ok, so I just mentioned it).
9. There is no global find on the iPhone. On my Treo, I can type a name in the global find field and it will find contacts with that name or emails with that name or tasks with that name. There is no global find on the iPhone. Unacceptable.
So that's the short list, but I could go on.
When I became aware of many of these limitations on the iPhone, I started looking for workarounds. For example, what if I put my many memos and tasklists on the web somewhere? There are sites that do that kind of thing. Safari on the iPhone can have up to 8 pages open at once. I could dedicate one page to my tasklist, one to my memos, etc. In reality, though, Safari's cache on the iPhone is set so low that whenever I started a new safari window and went to a new webpage, it effectively flushed any other pages I had open. When I came back to the previous page, it would be blank and I would have to reload it, which was slow (or not possible in the no-coverage zone of my workplace). So even though 8 pages can be opened at once, in real-world usage, only one window can be effectively used at a time. I gave up on trying to find workarounds for its deficiencies and I returned it.
Don't get me wrong, there were many things about the iPhone that I loved. It is beautiful. The screen takes your breath away every time you turn it on. The iPod functionality was phenomenal. Viewing your picture albums on the iPhone is just fun. Surfing the web was pretty nice too when you were in a Wi-Fi zone. The interface is elegant and intuitive and the device is very sleek and thin. My whole buying experience (I purchased from an apple store--walked in, walked out in two minutes with the iPhone) and activating it with iTunes was simple and enjoyable. Even returning it was easy compared to what I am used to with other carriers.
So while I did enjoy my time with the iPhone, the Treo's shoes are way too big for the the iPhone to fill. The iPhone has a ways to go before it can become my work-a-day, get-things-done device of choice.