Your thoughts reflect how I felt until I obtained one. And, for
single day trips to London you may be better of buying a day return to
zones 1thruN.

However, if you travel a lot within London it is very liberating. I
registered my Oyster on-line and added automatic top-up. Buses trips
seem expensive at GBP1.35/boarding. But the Overground and subway
journeys are reasonable.

Ditto! And you can use them on some local rail routes, for example to
Enfield, Chingford etc. I have no idea how you find out WHICH rail routes
though.

And one thing to watch out for, Network Rail often leave the barriers open,
inviting you to just walk though whilst forgetting to "swipe" your Oyster -
and then the ticket inspector does you for a penalty fair on the train!

Paul DS.