We just got back from an OAT tour to Peru. It was fantastic. It is perhaps a step above a Rick Steves tour in that your airfare, transfers and luggage handling are usually taken care of (we still pack light, but it sure is nice to have someone else schlep the bags from hotel to bus/van/airport, etc). This our 3rd trip with GCT/OAT - the other 2 were small ship cruises (50 passenger coastal ships). We have booked airfare through them (which at the moment you must do to get the free transfer from airport to hotel) and it has been competitive. We have also booked extra hotel nights, although you can get better rates if you do this on your own.
Tours have a great balance of tour activities and free time. We have found their trip guides (Trip Experience Leaders) to be excellent - good story-tellers, well-versed in the local history, very willing to make suggestions and usually offering an activity during your free time if you are interested.
While we travel as a couple, we have several friends who travel solo (male and female) and our OAT tours have had many solo female travelers, many of them repeat customers. They feel secure, and like that the pesky details like transportation and luggage are handled for them.
The Vietnam/Cambodia/Laos trip with OAT is on our radar, for sure. Have heard consistently good things about their Ultimate Africa tour as well.