When I got my Lumia 920 I was hitting comma instead of space about 80% of the time
. After a while it got better, although I still hit it occasionally. I switched from an iPhone and, as others mentioned, the position of space bar is slightly different, so I had to police myself to touch a little more to the left. I tried using the left thumb, but that didn't work very well.
Overall I really like the keyboard, though. When I looked at it the first time I thought it would be a pain to get used to it, but it was much easier than I expected.