I just used a Nexus One for 48 hours (thanks Pierre). Here are a few quick notes/thoughts from the perspective of an iPhone 3G user.

Getting started

• Take the SIM card out of the good old iPhone 3G, put it into the unlocked Nexus One, and voilà, you are done (sans 3G since the Nexus One does not support AT&T's 850 MHz 3G band)!
• The phone asks you to log in with a Google account. Since my contacts were synced with Google already, all that information was transferred right away to the phone.

First impressions

• Build. The Nexus One looks beautiful and solid.
• Display. The iPhone's clearly pale in comparison (especially resolution). On the other hand the OLED screen doesn't perform as well in plain daylight.
• Speed. The Nexus One is snappy. I don't have an iPhone 3G S, but the Nexus One is definitely way faster than the iPhone 3G (which often feels impossibly sluggish).
• Android. The OS requires some getting used to, but you quickly figure out how to do most operations, especially if you are a bit computer-savvy. The four buttons at the bottom of the screen are not as elegant as the iPhone's single button, but they usually make sense and you get used to them quickly. I had very little difficulty adapting.

 The cool stuff

• Multitasking. A few examples:
• Upload pictures in the background.
• Play Pandora while using any other app.
• Switch between a few open apps in an instant.
• Notification panel. It tells you quickly of things like new emails, tweets, completed downloads, and more. The iPhone needs UI improvements in this area.
• Text completion. It is different than the iPhone's, and often better as you can quickly pick words from a list.
• Web browser URL completion. Android seems to use something like Google Suggests to help you enter URLs.
• Voice input. This works only part of the time, but it's a good start and it's available in any text field.
• GMail. It is excellent and supports starring and conversations, unlike the iPhone's mail app.
• Navigation. I didn't have time (or 3G) to really try it, but the GPS navigation app must be quite cool.
• Google Voice. No need to fight with Apple here, Google Voice is fully integrated.
• Camera flash. That seems pretty basic, but it is not a feature of the iPhone so far.
• Programming. I haven't tried to program anything on the Nexus One, but I like the idea of being able to use Java (or even Scala) instead of Objective-C.
• Openness. Open source OS. Multiple hardware vendors. No approval process like Apple's (you can even change the default web browser on Android). This has some appeal.

The OK stuff

• Web browser. Safari on the iPhone is still better overall.
  
• App market. It is easy to use, but is nothing to call home about.
• Bluetooth. My Bluetooth stereo headset was setup quickly and usually worked ok, but:
• I had an issue whereby at some point the sound started coming out of the phone speaker again in spite of the Bluetooth connection being active. Later things started working again.
• My Bluetooth headset suffers from the "jeans pocket" syndrome: if you put your phone in your pocket and start walking, the sounds starts breaking (yeah I know, crazy). This happened more with the Nexus One than with the iPhone 3G. This tells me that the Nexus One's Bluetooth signal might be weaker.

The not so cool stuff

• Apps. Clearly there are less apps and of lesser quality. A few examples:
• I missed OmniFocus, Jott , and Instapaper / Read it Later.
• The Facebook app (not that I use it much) is still inferior.
• Twitter clients clearly don't beat Tweetie on iPhone (Seesmic was ok though, and I hear that Twitdroid is pretty good but I didn't want to spend money to try it).
• Multitouch. There is no multitouch (especially no "pinch"), and the Nexus One screams for it. UPDATE: Google has just released an update with multitouch!
• Copy/paste. This is pretty bad compared to the iPhone: text selection is difficult and not available everywhere.
• Media. The audio and video players are not as easy to use as the iPhone's. There is no built-in support for podcasts (you can try Google Listen for that or other apps). (I used doubleTwist to copy some mp3s and videos to the phone and it worked fine.)
• Trackball. The trackball is almost exclusively used to move inside and between text fields/areas. My feeling is that it does a poor job at this and Google should just abandon it and use another way of navigating.
• Screenshots. There is no built-in screenshot capability. The iPhone does this out of the box. Apparently you have to root your phone to install 3rd-party screen capture apps. This would have been useful for this post.
• Landscape mode. This is only possible in one direction instead of two, unlike the iPhone. It took me a while to figure out why landscape mode worked some times but not others.

Is it better than the iPhone?

I have no complaints about the hardware except for the trackball (concept and implementation), and the CPU and display give Nexus One the edge until this summer when Apple hopefully releases a new iPhone.

As for the software, it's not easy to determine a winner. Clearly a lot of thoughtful work went into Android, but it doesn't feel as polished and slick as iPhone OS. Often the phone feels more like a regular desktop computer. This can be good or bad depending on your perspective.

On the apps side, iPhone still wins hands down, but the Google stack of applications, including GMail, Navigation, and Google Voice, works best on Android, and that might win the hearts of some. Anyway there is little doubt Android will soon have enough good third-party apps that this won't be as much of a problem anymore (except maybe for games).

Most likely my next phone will still be an iPhone because of the apps and ease of use, but this short Android adventure was quite refreshing!