I volunteered with EHN for 3 weeks in September 2012. It was my first abroad volunteering experience, so it was pretty scary and was all new to me.
First thing that attracted me to EHN was the cheap program cost. That's pretty much what you initially look for, right? EHN beat everywhere in terms of cost - they're one of the most economical, honest organisations I have ever encountered.
The projects they provide are in areas of real need in Nepal, and are well-evaluated by EHN staff to ensure they're safe, ethically sound, and so you know what to expect before you start. Some of the projects were quite difficult at times; sometimes the local project staff were hard to work with, but that's essentially real life. That was the most important thing I learnt during my experience with EHN: it was real life out there, and it's not consistently easy and joyous. Really, the same applies to all projects worldwide. If you don't feel the real hardship of the community you stay with, you haven't been given the true experience that you perhaps paid a lot of money for. EHN seemed far more committed to providing a genuine experience than any other organisations like Frontier or ProjectsAbroad, and at a much much much lower price.
I couldn't recommend EHN enough for your Nepal visit.
The organisation is still young, so there were some minor hiccups. When I visited in September 2012, the organisation had experienced an unexpected boom in volunteers and the staff suddenly had a heavy workload to deal with.
I still check in on EHN at present and they have since recruited an extra group of people to help them cope with the demand and keep up the good things they do.