I've heard quite a few comments saying yes it is night and day and other saying they were actually disappointed and it wasn't as good as they would have though.
The idea of having to use an external amp really doesn't appeal to me at all for portable use, I don't think it's ever worth it.
The reason so many are coming up in the FS section is finally there are some better alternatives.
For example the HiFiMan HM-801 and it's little brother the 602. Sure these don't offer the big capacities of the iMods/DIYMods but that is not what they are about. They are purely designed for SQ.
The ease of having an all in one unit as well as better SQ is the main reason many users are moving to the 801 as opposed to their iMod set up.
If you want the best sounding large capacity player the only real option is the iMod but if you want the best sounding you have to look at the 801 (and maybe the 602).
Keep in mind these are just my opinions based largely on what I've read and quickly auditioned.
Personally I'd never consider an iMod but that's because I would never lug all that around just for portable use. An iPod 5.5g sure, but not an iMod.