I lived one year in Tanzania doing my Volunteer Service close to Moshi and already knew Charlie. So I decided to do my safari to Ngorongoro Crater (for one day) and mountain climbing (Marangu route) with Kilimanjaro Vikings. And I regret nothing. I did the trips with my brother and a friend and we had a competent guide with us on both tours. Charlie knows not only extremely well about nature and animals, you are also always in good company.
So, if you are still searching for a good organisation, I can only recommend you Kilimanjaro Vikings.
Have fun and enjoy Tanzania. It is worthwhile!