I've been using them for 2 years and they work like a charm, if you know how to properly place limit orders and don't mind fighting with the bots a bit.
I think the system works well: people who do bullshit chargebacks probably don't get to keep the money. VirWox probably puts up a good fight on this issue. PayPal is extremely profitable, and used everywhere. That's because they're smart about giving chargebacks.
For instance, I recently used a PayPal debit card at an ATM in a foreign country, and even though it spit out no money and the receipt said that I withdrew "$0"... "Unable to Dispense"... the amount got deducted from my PayPal. I was sweating bullets until I called PayPal and the gave me back the money almost instantly.