There are some extra keys, but some of them seem hard to use well (the big red triangle keys at the top of the thumb clusters) and other things that sound great in practice, like "hold" = modifier but "tap" = insert some character, in practice have turned out to have more annoying rough edges than I had hoped.
It's very well-made though, the configurator works good, etc. Seems like a high-quality gadget, but I wish I thought a bit more about what I would do for arrow keys, special characters, modifiers, etc.
I found the Kinesis thumb clusters a lot more usable when I used to have one, but I really wanted to try a split keyboard this time.
Build quality is also much higher, and the attached wrist rests really help after standalone ones floating all over my desk for years.
Some folks make do with 4 thumb keys.