I've been using my Oystercard for a while now. Until recently though, I just loaded it up with a monthly (student) travelcard each week. However, with University now over for the summer, I thought I'd save myself a bit of money and get the bus instead. I was going to just get a monthly bus pass, but seeing that Pre-pay now works on buses, at 70p a go rather than the cash fare of £1 (even when avoiding zone 1), I thought I'd give that a go, as I don't get the bus every day.
The card already had just under a fiver on it, in case I needed to go outside my travelcard zones, so I tried jumping onto a 253. It didn't work, the reader displaying 'travelcard exp[ired]' instead, so I had to hand over a quid instead. However, I persevered, and topped up my card with another £20. From then on it seemed to work, even on Routemasters, where the inspector has a special reader.
However, there have been a few occasions now where the Oystercard reader on the bus has been out of service. The bus drivers simply just to wave all the Oystercard-carrying passengers through when this happens, assuming that people have valid bus passes/travelcards. This means that pre-pay customers basically get a free bus ride. You could, of course, pay the quid cash fare instead, but then why should you have to pay an extra 30p because their machine is broken? Of course, you could always carry a book of savers (costing 70p each when you buy a book of six) around for these occasions...
The other thing I wanted to test was what happened when you went on enough buses in one day that it would have been cheaper to get a one day bus pass? A one day bus pass is £2.50, which is the price of three 70p fares plus another 40p. I was hoping that the system would be sophisticated to charge just 40p for your fourth journey and then stop charging you, however, when I ended up catching four buses the other day, I discovered that this doesn't happen, you just get charged 70p every time. I can't really complain, they didn't say anywhere that you'd be able to do this, but if they want people to use their Oystercard exclusively in future, this would be a nice feature. Taken to extremes, you'd be able to just top up your card with a couple of hundred quid at the start of the year, and a computer somewhere would work out the cheapest combination of tickets.
Using the Oystercard on a combination of tube and overground rail still causes a few problems, but there's probably not a lot that TfL can do about this in the short term.