We just got back from an 8 day safari and wanted to post our thoughts for other travelers (especially for those of us who had no idea what to expect when going on safari). We used Good Earth Tours, our guide was Rafael (mentioned a few times on this site in other threads).
1. In terms of locations and what you'll see, I think most tours will provide you with similar experiences. However, the approach can definitely vary. Our guide, Rafael, was amazing. He spotted animals and even insects from farther with just his eyes than we could spot with our binoculars (even when staring for several minutes). He added background and color to nearly everything we saw, from the interaction of the wildebeest and zebra to the termites and their mating habits. He also had a natural enthusiasm which made the information so much more interesting - you could tell he really loved his work. Also, he approached every situation with respect and calmness - he taught us to wait for the most interesting things to happen, rather than just driving fast to a spot that may be rumored to have a lion (we saw plenty of other tour guides take that approach, sadly). If you decide you use Good Earth, I highly recommend Rafael.
2. The price was on par with other tours we reviewed.
3. The food at both the lodges and the 'packed lunches' we received while on the game drives was average or below. This was probably the only negative part of the trip. It wasn't unbearable and no one got sick, but it's not something you would look forward to necessarily.
4. Side trips arranged by Good Earth (we hiked Oldonyo Lengai and also went to Olduvai Gorge) were not necessarily of the same quality as the main tour. This is mostly due to the fact that the guides for these separate excursions sometimes didn't speak English or weren't very knowledgeable. I'm not sure whether Good Earth can be blamed for this, but it's just a warning.
5. The lodges were all very impressive (structurally and facilities-wise) so don't worry - you'll have internet access much of the time.
6. Bring a fly swatter! Tse-tse flies abound in certain areas.
7. You will be taken to some souvenir shops (the idea to help feed money back to the community which I think is good), Just be aware that the prices they start with are usually about 10 times higher than what you might expect before the negotiation starts. For example, a t-shirt, wood mask, and bracelet had a total cost of $250 US dollars, but we negotiated the price down to about $25.