If somebody really wants to gain access to your iPhone, they'll get in.
Phone thieves (and mischievous friends) can easily figure out your passcode just from looking over your shoulder or tracing over the oily smudges on the screen. And if you're using popular passcodes, birthdays, and addresses on your iPhone, you might as well disable lock screen security entirely.
You can use a blank keypad to protect your passcode from lurking bystanders, but for true protection, your passcode should be changed regularly. But even if you're not lazy about it, sooner or later you'll lose track, forget... more