I finally cracked the other day and bought the Galaxy S2. And after a lot of playing around with it here's a few tips & tricks:

Ringtones and Notification Sounds
Store ringtones and notification sounds in media/audio/ringtones and media/audio/notifications respectively. This way they will not show up in the music collection.

Screenshots
To take a screenshot, press the Home and Power keys together and let go. You'll see a screenshot notification popping up, and the capture is store in the ScreenCapture folder.

Activate Voice Commands
Press the Home button twice. If it is the first time you use Voice, it will take you through the setup.

Quick Scroll Through Home Screens
Slide your finger over the dots at the bottom of the screen. This also works with the App Drawer.

Number of Home Screens
By default the Galaxy S2 has 7 screens. This is also the maximum number. To remove/add screens, press Menu > Edit. You'll see thumbnails of the current screens. To delete a screen just drag it onto the Remove icon. To add a screen, click the +.

Resize widgets
This only works on some widgets like the default weather one. Press and hold the widget. Once you get the haptic feedback let go. A grid is displayed indicating the maximum size and in the bottom right is a sizing handle. Drag the handle to the new size, then tap the widget.

Swipe Contact to Message/Call
Open Contacts list, find the contact and swipe it to the left to message, or to the right to call.

Video Lock Screen
When watching a video it's easy the accidentally press a button. Start the video and then press the power button. You'll see a padlock to indicate that the touchscreen is locked. Press it again and it's unlocked.

Task Manager
Hold the Home button to access the Task Manager and recently used apps.