We have just returned from a 2 week trip to Tibet which we organised through Tibet Namchen agency. Whilst it was overwhelmingly positive, we did have some problems that others might like to raise before booking their own private tours.
Firstly, it's good to know that agencies can request specific drivers and guides for tours. We would recommend that our guide (Tartar) and driver are only used for large group tours. It was clear that neither had read our private tour itinerary beforehand and both of them were forever anxious to hurry us along as if we were on a group tour that they were in charge of. They didn't seem to understand that with a private tour we were in charge of our own itinerary as it was one that we had personally devised. We both found it exhausting to constantly pull out the itinerary and insist we follow it. Fortunately we had written down exactly how long we required at each sight to avoid being rushed through at a time convenient to the guide & driver.
TIP: Detail everything! If you want 3 hours at a site to take photos, state it. Likewise if you want to walk up to the fort overlooking the monastery, state it. If you want sunset or sunrise photos at EBC, state it on your itinerary, if you don't, the guide could refuse to allow it. 'Not allowed' seemed to be our guide's favourite phrase!
Our driver, in particular, was bad tempered throughout the tour culminating in him banning the use of the cigarette lighter socket to charge our iPhones by stating that it was affecting the car's cameras and GPS. This was after 4 days' use without any problems. I had never heard anything so ridiculous in all my life. If anything was likely to affect on-board surveillance equipment then it would be the mobile phone signal not a simple electrical charge. This is why aeroplanes ask for phones to be put into flight mode yet have no problem allowing passengers to charge their phones via USB. Also, if this was the case why hadn't the cigarette charger been disabled? This was a purely spiteful action which even the agent in Lhasa could not reverse and could easily have ruined the trip with a constant search for restaurant charge points had I not had the foresight to bring along a spare battery pack. It was at this point that the driver lost all hope of getting a tip from us.
Regarding the tour itself, there were 3 sights which we didn't see: Shegar zong (the local police wouldn't allow us access) Gyantse zong (closed for the season! Surely this could have been foreseen) and Yumbulagang (closed due to construction work - this should have been foreseen too)
These were all sights of a personal interest to us that many standard tours don't include so we were left wondering to what extent our private tour differed from a group tour. Fortunately, when our guide tried to tell us that the Drak Yangzong caves were also impossible to visit, I had the foresight to speak to the agent and he was overruled. He then spent most of the next 2 days sulking like a teenager which we found tiresome. Only after the visit, did he reveal that he had only ever visited the caves once as a small boy and that he didn't remember much about them. We assumed that his reluctance to visit the caves stemmed from an unfamiliarity with the route or a reluctance to undertake the hike. As it was, apart from a careful bit of driving to get past a construction vehicle in Ngadrak, everything went smoothly and he ended up being more enthusiastic about the caves than we were.
Hotels outside Lhasa.
Many agents have specific hotels which offer them preferential rates but many of them are dire as Tripadvisor will reveal. With a private tour there is nothing to stop you choosing alternatives. However, in practice we found it made little difference. Despite agreeing an itinerary with the agent months in advance, we found out on the day of departure that no reservations had been made whatsoever. To make matters worse our choices were either suddenly declared full (in November!?!) or the guide conveniently stated that they weren't answering the phone to allow a booking to be made or the guide didn't know where they were. In the end we agreed to accept anything with hot water although half the hotels we stayed in didn't have any. We knew beforehand that this could be the case, so accepted it with good grace but still couldn't understand why no reservations had been made beforehand especially as we had prepared the itinerary months in advance.
Having said all the above, we still feel that a private tour was right for us. We took an extra day to acclimatise en-route to EBC and thus avoided altitude sickness and got to visit EBC twice: at sunset and sunrise. We visited Phuntsoling monastery which is surely one of the most dramatic settings in the country and we also explored the Drak Yangxong caves. These are things that a group tour would not have included. (Most group tours rush back from EBC to Shigatse at dawn the next day and many agents tried to fob me off that Rongbuk was the closest I could get to Everest. Others said I could go as far as the site of the tented camp (4km closer) whilst hardly anyone mentioned that the site of the tented camp was only half way and that you could go an additional 4km closer in your hired vehicle or on foot. (See Lonely Planet Tibet)
So to summarise, a private tour is well worth doing but be prepared to do battle with the guide and driver over the timings on the itinerary and be ready to phone your agent in Lhasa in the event of their intransigence as we did on 3 occasions during our 8 day tour outside Lhasa.
Good luck and enjoy this amazing country!