Is it fairly straightforward and comfortable to drive to the colonial cities
From my experience earlier this year, I would say:
- Mexico City airport & around: as I said, I wouldn't do it again; the ring around Mexico City is so busy and the signposting is terrible, while there are excellent buses to take you to the main cities (even from the airport directly)
- Colonial cities (Morelia, Queretaro etc): the main highways are OK (toll roads often), but it is difficult to get into the city centers; you also have to consider parking (I left my car a few times at shopping malls outside the center and took a minibus or taxi in). Actually you need a car when you want to see as many colonial city WHS as possible, but for a more relaxed trip I would suggest public transport
- Yucatan: very easy to drive, much less traffic
Having a car surely added to my stress levels during that trip, but without it you can't see so many WHS in such limited time.