I just wanted to plug Kogongoni Lodge. We stayed there about a month ago. It is between Arusha and KIA and has a website. It gives some of its profits to benefit a nearby home for mentally disabled children.
Kigongoni was by far the nicest place we stayed. The cottages (set in a lush tropical landscape) are beautiful! They are very private with their own little patio or deck looking out to the valley below and Mt Meru(?) in the distance.
The food was great and they let us stay in a room until 5 pm without charging us extra because our flight didn't leave until 8 pm.
Also the internet was free and for Tanzania, pretty fast.
One thing that could be a problem for some people are all the steps down to the cottages from reception and dining areas. There are some cottages that are closer without quite as many steps.