I had a play with an M600i today when I went into the shop to look at a Nokia E61. While I loved the fact that the M600i was narrower, I really struggled with the keyboard for the minute or so I had with it.
How have people found the keyboard? Did anyone find it unusable initially but get used to it? Do you hit directly on the side of the buttons, or put your thumb in the middle & roll across?
Also, when working with Quickoffice, can you switch the phone into landscape mode for when using an external keyboard (stowaway drivers should be coming I believe)?
If I were happy with the keyboard, it just comes down to how much I want WiFi (which is not essential, but nice), as I quite like the dimensions (well, the width really) of the SE over the Nokia.
A bit late to pick up this post but my answer is - it's fine. I actually find it much more accurate than my old P910i and I am even beginning to use predictiuve text (which I have never used anywhere else before) because the implemnattion is so good.
In answer to your question, I tend to hover my finger over the button and then tilt it so it presses down on one side or the other. Of course the very occasional mistake is made but I don't think I've ever pressed the wrong half of the key in error.
I went for the M600i over the P990i because of size and weighty and my biggest concern was that I would sacrifice usability with the keyboard. I bought a 'phone on 30-day approval and decided to keep it within the first twelve hours - really, the keyboard is not a problem.