I just recently did the Tsum valley trek in May/June 2014 and I must say it was quite remarkable!!! I think if you're looking for an off the beaten path culturally rich experience it's the way to go. I ended up going solo with my guide and porter and it was a zen like get away. I did this trip as a way to have some time alone and to reflect and I got just that. It was really nice visiting all the villages along the way and to also visit some of the monasteries. The views and scenery in the valley left me speechless at times. I was worried about the monsoon season approaching but it was not an issue, we had two days of rain during the 20 day trek and both times we got a village just in time to not get caught by it. As far as difficulty it was fair, nothing that was ever dangerous in any way but did have a few days of a good uphill grind.
In my whole time I only ran into 3 other tourists, which is what I was looking for - to really get away. The accommodations along the way were pretty basic but it was always possible to find a room with a bed, cold shower and a meal. There was by no means any options for a luxury stay but that's not the point of going to Tsum valley. I ended up going with AME treks and made my arrangements before leaving Canada and it was a nice setup. The guide spoke pretty good english and taught me a lot about Nepalese culture along the way.