It does an ok version, as far as extract amounts go. It helps with the hops calculations when you change them. It's just a computer program though, so it is always good to post it up here on the forum and have some one with more experience take a look.
I did a conversion in Beersmith once, for example, that added two pounds of caramel malt in the conversion! That would not be a "good" conversion. Overall, though, it's ok.
You can tell the program what to use to replace 2-row, munich malt, etc, so it does have some flexibility built in.