I booked 3 day tours at click2gothailand.com The first one, River Kwai with trekking, was a stellar experience. Although there were 4 of us in the van, I was the only client with the trekking option. Not only did I manage to do the full day tour as promised (museum, cemetery, train ride in morning; then after a good lunch, elephant ride and bamboo rafting), they decided to upgrade me to Erawan Falls, as that was where my other 3 travellers were all located. A great day, in spite of a few bruises from rocks at the Falls.
However, the other 2 day tours were definitely 'hit and miss'. The 1/2 day D. Suthep/Hmong will afternoon tour was cancelled, due to lack of clients (I did receive a full cash refund, the next day) and the Pai day tour started out poorly (fast driving up and around mountain roads led to motion sickness), but ended well with a visit to the Hot Spring, the star of the day tour, and a solid lunch at a local restaurant in Pai.
Their tours are very well priced, but the quality is a bit 'hit and miss'.