I stayed in the private house for a week at the end of February 2014. As I was the guest of a friend I cannot comment on the value proposition. However, I can say that this was an amazing experience and if you have the opportunity to go you should not hesitate.
The house itself is huge, thoughtfully laid out so that every bedroom suite is completely private. The furnishings are perfect -- comfortable and elegant without being fussy. The views are incredible, in particular the sunsets, when you can see in the distance the Masai herding their cows back home.
The staff is attentive and accommodating but not at all intrusive: Levi the cook, Francis the waiter, Ziporah the house manager, and Jarat the housekeeper -- wonderful people, all.
The grounds of the entire Cottar's camp are beautiful. I can't comment about the tents because we didn't stay in them. I do imagine that they would be warmer than the house, as they are at a lower elevation. The pool is beautifully situated. The main dining tent does indeed summon up a twenties feel, and when I saw a lunch table being set out on the lawn just beyond, I felt I was looking at a painting from that time period.
Of course we went out on game drives, and with our Masai spotters we saw a lot of animals. We actually never went so far as into the Masai Mara National Reserve. Instead, we went out mainly at sundown and sun-up for drives not far from Cottar's -- I'd say we were never more than 45 minutes away from the camp. There were two virtues of not going all the way to the Reserve: 1) We didn't have to pay the daily entry fees of $80/person, and 2) We were not 'competing' with groups from other safari camps all looking for the same lions or whatever. We rarely saw another safari vehicle, and if we did, it was always a Cottar's vehicle. We were also fortunate to go on a nature walk near the property with a gentleman named Moses who has worked at Cotter's as a guide for many years and has an incredible knowledge of the medicinal uses of different plants.
Cottar's is clearly a good citizen/neighbor of the Masai, as well as being a lessee of the Masai lands. Cottar's visitors are charged a conservancy fee, which is directed to the local community. We visited a local Masai village and were then taken to their elementary school, where the principal told us that Cottar's pays half the teachers' salaries.
Cottar's has a little gift shop with a nice variety of gifts of uniformly high quality. Prices seemed reasonable.
This was a trip I'll remember all my life. Highly recommended.