Does replacing the mattress in a sofa bed make it more comfortable?

I have a traditional sofa sleeper that is very uncomfortable. I have seen a number of mattress replacements on line that offer memory form, are part air mattress, or just claim to be better than a traditional mattress.

I am concerned that the bed is uncomfortable primarily because of the bed support and no matter how good a mattress I get it is still going to be uncomfortable.

Has anyone had experience replacing their sofa mattress? Did it help? What kind did you get?