I use it as my only alarm clock. Has worked perfectly for the last 4 years for me.
Same here.
My only hiccup so far in 4 years of iPhone alarm clock usage was actually just this past Thursday. My alarm either didn't go off or I somehow turned it off without waking up or remembering it. But I still managed to wake up (thank God), randomly, on my own about twenty minutes later and made it to work on time.
However, if you believe the interwebz the iPhone is apparently "notorious" for the alarm randomly not working