In the past, for pre and post tour private excursions, we have relied on companies recommended by Tauck travelers, Trip Advisor, and Rick Steves. While that provides no guarantee, I have been very pleased with those we contracted, most recently in Frankfurt, Prague, Florence, Naples and Capri. All companies provided excellent, licensed guides, separate drivers and nice sedans or vans, typically Mercedes.
There have been some recent mention here on the forums about Tours by Locals. It sounded intriguing, so I did some research. It appears Tours by Locals is just a broker like Viator and many tour companies which rely on independent contract guides. While they may be fine, reading a bit farther on the guide info pages revealed that many (most?) are not licensed guides. That is a big deal in EU countries. An unlicensed guide is not permitted to lead you inside any government maintained historical or cultural sites. I find that a real bummer. The other negative, was that a significant number of guides also drove the car or van themselves- I'm not too thrilled with that.
What have been your experiences with Tours by Locals? Next year we will be looking for a guide for Athens and Delphi.