xbee Compatibility with other 3rd party manufacturers


I am seeking to develop a large network of sensors. The question of xbee s1 or 2, being compatible with similar devices from other manufactures has arisen. It was my hope that it would be the case but attempts with 2 other manufacturers(olimex and cirronet) have so far proved to be fruitless. Is it that i am missing a trick or are there indeed compatibility issues. can someone please shed some light.