I have some issues with DDK.
One problem is that (unlike a D2K kit) the conversion entails permanent physical alterations to an otherwise working Junior board, which is not good on a preservation level. The alterations may be reversible, but probably not by anyone except Mike's Arcade, since they don't make the steps public. It's not technically destructive, but it is a disfigurement.
Sound: I can't speak for anyone else and their preferences, but I couldn't live with the sound changes. Jumpman's running and jumping sounds are iconic, they're part of the game. Junior sounds... that just isn't right on DK!
Price: At $160 for the conversion, the surgery costs almost as much as simply buying a DK board, maybe even more if you can find a deal.
Options: if you just want to easily switch games, there are other non-invasive ways to accomplish that, like the Nintendo PCB switcher that Vector Labs makes. You could even just go with a 60-in-1 if switching games easily is the main priority. Even in their raw original form, it only takes a few minutes to swap a DK and a Junior board.
Lastly, I can't find the link right now (I will look later), but it was determined at one point that (unlike the D2K kit), there actually IS at least one confirmed difference in the gameplay when DK is played on Junior hardware. Specifically it has to do with the timer ticking at different rates. You can live slightly longer at the "000" before dying, for one thing.
So I'd be cautious about DDK! It's one thing for a collector, or an arcade operator, to go that route, but for competition it might be a problem. It certainly was on at least one occasion...