Hi, first of all sorry by my English.
I want to create a sensor mesh and I need to use RF. I think that the usual 433Mhz modules or the NRF24L01+ can help me. In principle I'm going to use NRF24L01+ because it has more features, like automatic ACK, CRC... I know that the NRF24L01+ can use up to 6 pipes, but I need to communicate with all sensors in range, in each node; this can be more than 7 or 9 nodes.
I'm thinking in use only one pipe in all nodes, the same pipe on all, with this I think that they will be able to broadcast messages. But doing this, I don't know if the NRF24L01+ will be able to use his auto-ACK, auto-CRC features or I will need to implement manually. In addition I don't understand how to use the NRF24L01+ libraries... 
(I have readed a lot, please don't think I haven't searched anything, including datasheet)
The questions are:
1-can one NRF24L01+ communicate with more than 6 sensors? (how?)
2-If 1 is true, can the NRF24L01+ use his auto-CRC, auto-ACK, etc...?
3-If 1 is false, can the 433MHz RF do the job?
It is very important to me, help please. Thank you so much!
I'm embarking on an NRF24L01+ project myself. So far, I'm leaning towards using MySensors and writing my own controller.
http://playground.arduino.cc/InterfacingWithHardware/Nrf24L01
See RadioHead: RadioHead Packet Radio library for embedded microprocessors for a mesh library that will work with the NRF24L01.
The RadioHead has a note about memory in the documentation. When using a full mesh network, a normal Arduino has not enough memory
Thanks to everybody. The question is solved: yes, the NRF can broadcast messages to "infinite" nodes. I think that the previous responses are talking about a NOT REAL mesh (tree structure).
Now I'm implementing my real mesh network with limited number of nodes and a proprietary protocol. (Mine)