It completes everything for you, activities, buying properties, buying stores, and the gang operations, I used it when there was a particularly annoying activity that I just couldn't get past, so I would say think about what activities you may have trouble with or what territories you'd have the most problems taking over. I think it should give you the achievement for it, as you still get the ones for 100% takeover of the district, even if you used the takeover assist. I normally do all the activities I can before I use that. I suppose if you're that worried about it, just do the activity and then use the takeover assist.
TimeSplitters 4: "Save the world or die laughing." Megaman Legends 3. Enough said.
You get none of the cash for strictly completing an activity. You get the cash per hour for any properties picked up/areas cleared. You get none of the respect. You get the respect modifiers for completing areas.
Diamonds are a girl's best friend. Dogs are man's best friend. The Rocket Launcher is my best friend.