I switched from a Microsoft Ergonomic to a Kinesis a month or two ago, because I wanted my mouse closer to avoid shoulder fatigue. I was pretty fast right from the beginning, but it took me a couple days to relearn space, enter, backspace, equals, and tilda. By day three I was getting comfortable. I still make mistakes, especially around hyphen vs equals, but they are getting less and less.

Now I'm trying to teach my brain to switch between Kinesis and the old way. The first few times I worked on a laptop I was furiously pounding on the spacebar trying to make the cursor back up, but even that has gotten easier lately.

So I'd say the learning curve is really pretty easy! Maybe harder if you don't touch type. A year ago I also tried the Truly Ergonomic, and that was a much harder adjustment.