I've got a Samsung Galaxy Note with SlideIt installed which works wonderfully. But I can't write with my pen anymore. So how do I switch VERY QUICKLY to the Samsung keyboard and back?
I had the exact same issue with my Note II. The best solution I've found is the app Quick Keyboard Switch, which gives you a notification bar icon you can tap after pulling down to bring up a dialog box to switch KBs. You can switch back the Samsung kb, and from there hit the handwriting input button to get back to pen input.
There's a little keyboard icon in the notification pane up top which appears when a text field get's entered. This can also be used to switch quickly. So It appears a third-party app is not necessary.