Yeah, now I see it was mentioned before, I didn't see that when I posted..
Unfortunately, I didn't use Nice so far, so I can't comment on speed. However, I can't say Scala code was too slow, except maybe for the compiler :-). Although Scala has no 'nullable' types as Nice, they can be emulated using the Option class (heavily used in the library). For the latter point, indeed, Scala can not inject methods into existing classes, but up to a point, they can be achieved using views.
An interesting comparison between Nice and Scala can be found here although it might be out of date (it considers Scala 1.0, while Scala 2.1 has been released recently).