I’ve had iOS 5 installed on my iPhone 4 since it became available shortly before lunch on Oct. 12. Here are some quick and dirty impressions after about a day of use.

Installation: Pretty painless. The download took only a few minutes over broadband Internet. The process of backing up and restoring my iPhone took a fair amount of time — more than 30 minutes.

iCloud: Upon turning on for the first time after the OS update, the phone asked me if I wanted to sign up for iCloud. I accepted. The process took only a few minutes, no doubt shortened because I already had an iTunes/Apple ID account.
I was able to use the set which parts of my iPhone’s content I wanted to back up in the cloud. It doesn’t take much to start gobbling up the seemingly massive 5 GB of backup space. It makes sense, though: My camera roll items are almost 2 GB to just to themselves.
I still haven’t fully explored the capabilities of iCloud. Once I’ve done so, I’ll report back to you.

Notifications in general: I love the new notifications screen. With a swipe of the finger from the top of the screen, I can call up the weather and stocks info without switching apps.
If I’ve skipped calls, text messages, reminders and other such messages, they all queue up there for viewing.
You can set which programs get to live in the notifications screen and how they alert you.
The notifications screen is also available in your lock screen but only if there are new messages/alerts to show you. You’ll see a ridged tab-like icon near the clock, which you can drag down to reveal the notifications.

Messaging/phone call notifications: When I got a text message whilst setting up my phone, the message innocuously dropped down from the top of the screen in a small banner and discretely retreated a few seconds later. Clicking on the message as it appears or selecting it later from the notifications screen will invoke the messaging app for me to respond.
If you prefer the old-school, intrusive text message notification, you can choose “Alert” under Settings > Notifications > Messages > Alert Style.
Phone calls, calendar items and timed reminders have the intrusive “Alert” notifications as a default.

The app formerly known as iPod: Apple has split the music and video portions of the former iPod app and called them — you guessed it — Music and Video respectively.

Newsstand: As the name of this app suggests, this is a Books-style app for newspapers and periodicals. Currently, the Canadian app store has a very limited number of titles for sale. The only icon my Newsstand contains is one for The New York Times, a virtual copy of the standalone NYT app I already own.

Camera from the lock screen: It took me a few hours to figure out that you have to double-click the home button to access the native Camera app from the lock screen. (It’s the same process you’d use to access Music controls when your phone is locked.)
You won’t have access to the full contents of your camera roll until you unlock the phone.

Auto-lock while using AirPlay: Some video-streaming apps (including Livestation and the National Film Board, for example) used to prevent the phone from locking itself while playing. However, this isn’t happening anymore when streaming to Apple TV using AirPlay. If you have auto-lock selected, the app stops and your phone locks after the set time interval elapses. Seems like a bug.
In contrast, the phone does not lock if the video is streaming through iPhone itself.

Other considerations: To allow for iCloud, some iOS 5 features and some feature exclusive to the upcoming iPhone 4S, the following may require a software update.
• The OS on your Mac
• iTunes
• iPhoto, Aperture, iMovie

So those are my initial observations and impressions of iOS 5. Check back in a few days to give me a chance to put the operating system to the test under extended, everyday use.