Even though I've bought every generation of the iPhone on the first day it went on sale, I've never been able to use the device as my primary phone for the entire length of time between the new releases. I left the first-gen iPhone for the HTC Advantage and then the AT&T Tilt after a few months, the Sony Ericsson Xperia X1 took me away from the iPhone 3G, and most recently I un-SIMed my iPhone 3GS in favor of the AT&T Nexus One. I've purchased other phones along the way as well, but these are the ones I attempted to use as my primary phone because of their compatibility with AT&T's 3G network.
The Nexus One had the most chance of "sticking" because it bests the iPhone in many ways (faster browsing, bigger screen, higher resolution, more customization, seamless Google integration, system-wide voice-to-text entry, better camera, various niceties in Android, etc.), but I still went back to my 3GS. I know that the iPhone is not the hands-down best phone out there, which is why I always end up leaving it for something else, but what I finally realized is that I keep going back because it's the best phone for me.
The Nexus trumps the iPhone in a lot of ways, but I learned that those ways aren't really that important to me. I mean, the higher-resolution touchscreen is great and Gmail is truly amazing on the Nexus, but I still have to zoom in to read web pages (just like I do on the lower-res iPhone) and Gmail actually works just fine on the 3GS. It isn't as good, no, but I can still label all of my email and receive/send messages like I want to.
The iPhone doesn't offer a unified inbox, but it still has just one Mail app. On the Nexus, I had to use the native Gmail app for my personal email and the regular Email app for my Pocketables email. I know I wouldn't have to do this if I just used Gmail to handle my Pocketables email, but I don't. I shouldn't have to change the way I do things to suit the phone, anyway. So while it's not as easy as it could be to switch between accounts in the iPhone's Mail app once you're in the inbox of an account, it's still just one app to deal with.
I also like how the iPhone handles zooming in the browser. It doesn't reflow text at any zoom level the way that Windows Mobile and Android web browsers do, but what it does do that I really like is zoom into columns perfectly so that both the text and the image are displayed with a double-tap. Opera Mobile and Android's browser both zoom into and reflow the text on a double-tap too, but they cut off the image. I can also usually read text on an unzoomed page on the iPhone, whereas text is often completely unreadable on an unzoomed page in WinMo or Android.
Because the iPhone doesn't reflow text with pinch-zooming, however, websites that utilize wide single columns (like forums) are difficult to read. This is where WinMo and Android have the iPhone beat, as the text in these wide columns are reflowed perfectly on these platforms, but I still give the advantage to the 3GS because I visit multi-column sites like blogs more often than any other kind of site. So the iPhone is still the better device for me and way I surf the web.
Another area in the web browser where the other platforms are technically better than the iPhone is full-screen mode. iPhone OS doesn't have it. The status bar on the top and the navigation tray at the bottom are always there.
I understand how having these elements disappear the way they do in Opera Mobile, Android Browser, and other browsers is great because they display more content, but again, for the way that I surf the web, I actually prefer having the top and bottom bars there all the time. I don't like not being able to see the time, battery, and wireless status when I quickly glance at the top of the screen. I also use those navigational controls a lot when I browse (especially bookmarks, share page, and new window), so having them accessible at all times means less tapping, which of course means quicker access to what I want. An extra tap may seem like a small thing, but it adds up: wouldn't you rather tap the screen 50 times than 100 to achieve the same thing?
The iPhone was criticized for a long time about its inability to cut-copy-paste but when Apple finally added the feature, it was done very well and very intuitively for a touchscreen device that relies on big finger pads rather than fine stylus tips. The magnifying glass icon and grab points were items that I didn't really appreciate until I had to use the wretched trackball on the Nexus One, which is necessary for precise cursor control and accurate text highlighting. I used to think that Apple was so ridiculous for showing off its implementation of such a basic feature, but now I think it's earned those bragging rights.
On the Nexus, you have one shot at highlighting the text you want; if you do it wrong (not hard to do using the trackball or your finger), you need to tap the menu key, then the More icon, then the Select text option again. Three taps just to put the device into "copy mode" is silly. Oh, and did I mention that you can't copy text from an email unless you use the browser or you're in an active text box? Cut-copy-paste can easily be improved with a firmware update but for the time being (and for the past few years), the way it's done in Android is not as good as the way it's done in iPhone OS.
I won't even bother mentioning the App Store as a reason for me to stick with the iPhone because the quality and selection of apps are unparalleled. Period. Even when you filter out the dumb apps that make you question the intelligence of mankind, there are still more apps that are better in the App Store than in Android Market, Windows Marketplace, Ovi Store, and so on. And although the current top phones are all great devices on their own, the apps make a big difference.
Other minor things I like about the 3GS are its native screen capture function (no extra software needed, just press the home and power buttons at the same time and a screen shot appears in the Photos app), hardware ringer switch (faster to mute/unmute phone than using an Android widget or swiping the volume on the lock screen), less aggressive auto brightness (the Nexus adjusts the screen brightness so frequently that it became annoying; I eventually disabled auto brightness and just controlled it manually), and the fact that you only have to tap a button to answer a call (rather than swipe the screen like in Android).
So although I wouldn't argue with someone who said the Nexus One was a better primary phone than the iPhone 3GS, it just isn't a better phone for me. What makes a phone great to one person could be completely overlooked or unappreciated by another person, and if you've given other devices a fair chance and didn't just dismiss them because of their brands, then what you think is all that matters.
So maybe the iPhone isn't the right phone for you, but I know now it's the right phone for me . . . until the so-called iPhone HD comes out, of course.
Note: I'm still working on a series of objective iPhone vs HTC HD2 vs Nexus One vs Nokia N900 comparison articles, so don't let my personal feelings about the iPhone prematurely sway your decisions. I will still continue to want, buy, use, and review devices running on different platforms (HTC EVO and Dell Streak, where are you?).