Spent four nights at Mara Siria just before Christmas 2011. Our tent #6 had a wonderful view looking over the Mara Plain just like the main camp. In its own private location, the tent was fairly well appointed with a comfortable bed and an attached ensuite with wash basins, flush toilet and a shower. There was power at the tent for lights during the night with hot water bottles to take the chill off cold nights. Given 20 minutes notice the staff would prepare hot water for a shower.
The first few days of our visit saw the camp relatively quiet with only about three of the tents occupied, the last few days saw 36 new guests arriving. Considering the two different dynamics at the camp the staff were able to react and keep the same high level of service. The food was tasty and the service quick.
Some reviewers have had mixed reviews of the location, outside of the park and up a rough steep road to the top of the escarpment. If you think you would be put off by this, then probably it would not be the best place to stay, that being said the views as noted earlier are outstanding and you get to see wildlife that others don't by climbing that hill. On different trips we were treated to herds of about 15 giraffes, many dik diks, impalas, baboons and jackals.
The little plunge pool that they have installed is cute and has some fantastic views and really any place at or near the Mara is heaven. If possible, try to get a tent near the escarpment that permits great morning and evening views. All in all, Mara Siria is a very good camp and would be a good choice for travellers in the Mara.
As far as the Mara, it is hard to go wrong, December is a bit tricky as the grass is a bit high, but still lots to see and yes we were fortunate to see all the big five... :)