I've never taken their tours, but read a lot about them when Amanda Ruggeri worked for them (she's the woman behind the website Revealed Rome). Here's her post from 2010: Walks of Italy Launches a Website, and More Tours

Since Ms. Ruggeri was involved with them I always figured they were a good outfit. (I used her site Revealed Rome for a lot of my trips.)

From first hand experience I too can recommend Context Travel, and also from research I've done I'm impressed with a particular Context Travel guide names Agnes Crawford. And she also gives private tours independently of Context Travel. Links are seen on this FT post. (And on that post there's also linked another FT thread with more recommendations for guides).