Although the staff were very friendly, our room at the Impala Hotel was pretty dreadful and I'd suggest staying somewhere else if your tour operator can arrange it.
The Impala Hotel looked nice enough when we arrived, but once we got to our room we were disappointed. After 36 hours of travelling, we just wanted a shower, a bed and dinner. Unfortunately, the shower didn't work; water just dripped down the wall and onto the bathroom floor (none of the water made it into the tub). There was a huge hole in the wall where the water had clearly been draining for a long time. There was also mold in the bathroom. It was one of the worst washrooms I've ever seen.
The walls between the rooms were paper thin. We could hear everything going on in the room next to us. We were fortunate to get an end room, so we only had to listen to one set of neighbours. If you know you'll be staying here, bring earplugs-- that should help.
Finally, the TV didn't work--possibly because none of the plug-ins for electricity worked in the room. Not a big deal, since we didn't go to Arusha to watch TV--but don't count on catching up on the news at home.
The one great thing about the Impala Hotel, as indicated in the Lonely Planet, was the Indian food restaurant. And the staff were also very friendly.
It is Africa, so we felt guilty complaining considering how most people live in Arusha. As we walked by other people's rooms we heard showers working--so they're not all broken. Just make sure you check yours as soon as you arrive and ask to be moved to a new room if you need one that works.