I have been getting mail all week without problems. However, I am not sure how fast it is since I don't test that very often. In another Post I did a test where if WiFi was on it would not Push but with WiFi Off it started working again. I don't think this happens all the time but I don't use WiFi that often.
Something new I did as a test. I added a dummy (but valid with no mail) POP account so that I could have it POLL every 15 min which I was hoping would activate Yahoo Push if it stop. After that my usage stats were matching my standby stats (like it was looping). My battery then went down very quickly. Since then I deactivated my POP account and it has been fine ever since.
I think Yahoo Push is just very on again off again. I like it better then using a POP account because of the battery life and I can clean it up easily via the web interface (can't easily delete all mail on the phone).
I just hope that some day they stand behind what they say in the iPhone User Guide:
Yahoo! provides free “push” email accounts. With a push email account, email is transferred to iPhone as soon as it is received by the mail server.
On Yahoo:
http://mobile.yahoo.com/iphone