I purchased a Visitor Oyster Card last summer and found it easy to use once I got the hang of swiping out at the end of a journey.
When I was back home, I phoned the TfL helpline to see how much I had left on it but they couldn't tell me. Data protection was quoted as the reason!
The chap did tell me that I had enough left on it for a bus ride or two though..
You can register the card on the Oyster site and see/update/top up your balance.