Yes I have completed them - and yes it is totally worth it, because not only does the showroom generate cash, but you get access to the Hotring Racer car (kinda like a NASCAR thing) which I highly recommend for completing the street races.
I just went through and did alot of the secondary stuff today. Here's some suggestions:
Vigilante: Use a tank. The worked so that the cars you're chasing don't go ten times your speed. Get all the secret packages and get the tank. When you finish level 12 you get 150 armor.
Paramedic: Do this one near Little Havana. All the people to pick up (for the most part) will be in the airport area, so you don't have to drive all over the island to find the patients.
I also finished the Pizza delivery stuff....How am I still only 72% complete?!?!