Went with Safari Makers in June, 2010 (Serengeti/Ngorongoro, mix of camping and lodges) based on a recommendation in this forum and the general "vibe" we got from initial communications. We can't say enough good things about them.
Our guide Mosses was terrific on every level - spotting game, educating us (we were with our 11-year-old daughter), managing the time with just the right balance, making adjustments as needed and/or wanted, and just generally being a great companion for our trip.
Our cook Juma is a saint given that he managed to find a way to feed the two incredibly picky eaters in our family, and with a great sense of humor.
Gideone, who handled the arrangements in the office is great too: personalized service, responsive, efficient, and incredibly patient as we fine-tuned the details. In addition to the safari, he booked our transport from Nairobi and then onward to Zanzibar.
We got several quotes, and Safari Makers wasn't the cheapest, nor were they the most expensive, but we feel like we absolutely made the right choice, especially once there... we didn't feel like we were missing anything not going with the higher cost option, and we could see some of the corner cutting elsewhere.
My one issue is that it would be nice if they could take credit card payments, although I'm not sure I would have been willing to pay the 5-7% surcharge other places in Arusha were adding to credit card payments. In the end, we simply invested in good money belts for all that cash.
Happy to respond to private messages with any more info.