I went on this holiday last July. It was absolutely fabulous, in no small measure due to the Just You tour manager. The boat is small enough to get to know everyone, and the crew were so friendly and helpful, and all took part in the enterteinments. And the food!! You may start the week thinking that you don't need to eat two three course meals a day, but believe me, your resolve will weaken by the seconf day! It was very hot going up the river - up to 40 degrees - but the boat was air conditioned and comfortable. For those who could stand the heat outside, the sundeck and (small) pool got a lot of use.
I booked all the tours. The tour guide was very good, and knowledgeable. Each town we visited was more amazing than the last. Rather a surfeit of cathedrals, but you don't have to visit all of them (but let the tour guide know if you want to do your own thing, so he doesn't think you're lost!) The best tour was to Salamanca in Spain - it was like being in a Shakespeare play. I took time off from the official tour there to spend time in the Art Deco museum, with an amazing exhibition of Galle glass. OK, not everyone's choice, but the building, refurbished to the original Art Deco style, is definitely worth a look.
The only criticism was that the pillows on the boat were not comfortable - if you have the space, and like a pillow that feels like a pillow, take your own. And the TV in the cabins doesn't get many English channels, so take your own radio/ipod if you need it.
But this was one of the best holidays I have ever had. I'm sure you'll enjoy it just as much.