I have also used Haskell in the past. Been using Nimrod for a couple of years now. I agree with defg: the differences are huge. But the great thing about Nimrod is how flexible it is. That's precisely where metaprogramming comes into play, while some features found in functional programming languages are missing from Nimrod they can be implemented fairly easily using Nimrod's macros. For example I have recently implemented syntax sugar for anonymous functions using macros (http://build.nimrod-lang.org/docs/future.html).

I do however find the imperative style of programming more efficient, when working with Haskell I always felt like I was fighting it. So perhaps Nimrod may not be for you, but it's still definitely worth checking out!