Well, it's based on the hp48 instead of the hp42, for one. Though that button layout is not that close to the 48, either.

Free42 is more faithful to the original 42 than this is to the 48. Free42 also has an infinite stack hack which is a nice bridge to those accustomed to the RPL.

This is annoying to installe and not really completed (this is a much much larger endeavor than the 42). Its also very disorienting at first until you figure out the UC

That said, its good enough that I use it on my swisimicro rather than my 48s because the screen is much better