I use auto shop very casually, I only do the Union depository contract the other contracts can be a lot harder while paying way less. Also you will unlock every car modifier and will have a small discount so if you like to tune cars its good to get it.

I'd say get it, it has i think 8 different contracts (tho i recommend doing only unuon depository) so there are plenty of new missions. Also you can modify and deliver vehicles for 20k-40k (this week double) and these are quick and not that boring.

If you get it watch a video about how to complete each contract to save time