We spent a few nights at Hotel Flamingo at the end of our trip. We traveled on a budget, and while relatively inexpensive, the hotel was the cleanest and safest feeling one we stayed in.
I cannot say enough wonderful things about the staff. Their help and suggestions were invaluable during our stay - whether it was booking a safari tour, arranging transportation, or recommending restaurants. One of our companions had an issue with her flights home, and the staff were so helpful and supportive while she untangled the mess with the airline company. Everyone at Hotel Flamingo were genuinely happy to help us (and I noticed this with other guests as well - it wasn't just us!).
I've spent a number of months in Arusha, staying in quite a number of hotels, hostels, etc. and I can easily say our stay at Hotel Flamingo was one of the best!