How far is the travel time form Dubrovnik to Vis? Is it worthwhile? Anyone been to Mlini outside of Dubrovnik?

To get to Vis you must first travel to Split (4.5 hour bus ride or at least 6 hours on a ferry). Catamarans and ferries to Vis depart from Split (about 2.5hours).
How much time will you have? Give me more info. (e.g., if you'd be able to include it in your itinerary, etc.) and I'll tell you how great Vis was.

Thanks Yrokshire! Our plan as of now is July 28-Aug 12. Dubrovnik 4 nights with day trips to Mlini and Makarska (if close enough) and rent car to travel to either Brac or Hvar for 2-3 nights and then on to Dortmund, Germany for a wedding.
Do you think Makarska could be a day trip?
And any idea of which is a better choice, Hvar or Brac island?
We are trying to get a flight to Munich since our trip will end in Germany, and go from Munich to Dubrovnik to begin our journey in Croatia. Any advice is certainly appreciated!

Yorkshire-
I forgot to ask if we should just do Vis instead of Hvar or Broc?
So...
Dubrovnik 4 nights
drive to Split (or bus?)
Catch ferry to Vis
Vis 3 nights

I haven't been to Mlini or Makarska, but you can stop in Makarska on your way to Split from Dubrovnik if you rent a car. (I believe it is much closer to Split).
The croatian national tourist board has a really helpful website. Hvar and Brac are easily reached from Split.
Brac has a few nice towns, a mountain with a fantastic view, a famous beach (with crowds to match). Hvar has gorgeous Hvar town and beautiful scenery. I have only been in May though, and in late July-August you will need to navigate the ver bust tourist season. My suggestion would be to stay in small towns with nice beaches (and drive to the bigger towns to see the "sights".)
Tiny Vis also gets crowds in August. It is gorgeous--rent a scooter and tool around the island through mountains and vineyards.