After a week I am getting along famously with Touch ID. I really like the ability to quickly access the iPhone without having to key in a passcode. Using the fingerprint is so much easier that I have reduced the auto lock feature delay from five minutes to one minute. Not only does this improve security if the phone is snatched out of your hand, it should also help battery life.
But which finger is which? By default, Apple calls them Finger 1, Finger 2 and so on. You can change the description easily. Just go to Settings/General/Passcode & Fingerprint, enter your code and then choose Fingerprints. Check which finger belongs where on the numbered list by briefly pressing the home button. As you do this the entry corresponding to the print you used will be highlighted.
Tap on the Edit button at the top of the screen and rename the individual fingers. If you have any problems in operation, as I did originally with my right thumb (the first one I set) it is easy to delete the print and start again.
Up to the arrival of the 5S I have been using a six-digit passcode (by turning off Simple Passcode in the Passcode & Fingerprints menu). Now that the passcode needs to be entered only occasionally and for special occasions, it is viable to choose a more secure alphanumeric string or add a symbol or two. The keypad has telephone style letter groups under the numbers for exactly this reason. However, this is probably overkill since most phone thieves are interested solely in turning a quick profit rather than in stealing your identity. It’s important, though, to remember to turn on Find My iPhone in the iCloud settings. That way you can tell Apple to wipe your phone’s data as soon as you realise it has been stolen or lost.