i am on ATT. for me, siri works about 80% of the time, regardless if i am on wifi or not. i would say about half of the time it works, it takes a very long time (1 minute+) to receive a response. the other half is usually pretty quick (3-5 seconds). i want to say that off peak hours are better. the times it does not work are usually long pauses followed by the standard siri msg stating she does not know what i mean, etc.
calling a number by voice dialing is excruciating - i had an apple 4 previously without siri and it worked very well, i am debating disabling the siri feature, but will give it another week to see if it is just mass congestion of the new devices on the market.