Just completed 3 weeks in Tanzania, all arranged through Achmed and his company Base Camp Tanzania. I could not be more pleased. The guides were the best, and we had no glitches worth talking about in doing Meru, the Serengeti, Kilimanjaro, and Tarangire. Achmed delivered everything hoped for. The accommodations were excellent (though L'Oasis in Arusha is terrible noisy at night) as well as the safaris and the climbs.
Achmed's prices are the best. As long as you are not expecting a luxury trip, you should be quite pleased. The vehicles we had were either relatively, or quite new. The tent camps were great fun and quite comfortable.
Our guide on Kili, Stanley, is one of, if not the best guide. He was extremely patient with us, fought (literally) to get us good camp sites, and has done Kili almost 200 times. Stanley had my husband and I at the summits of both Meru and Kili exactly when he wanted us there. He KNOWS these mountains and how to get you up there.
Achmed has been in this business some 13 years. He is half German and half English, so he understands American (and European) expectations. He is also typically very fast with the e-mail replies. Exceptions have generally had to do with Tanzania's erratic power supply.
Anyway, I can not recommend Achmed and Base Camp Tanzania enough. Just be prepared for some off-color jokes...