Every single problem that I have heard of people having has involved very large sums of money (over $50K).
This triggers them to look into what you are doing to make the money.
If they feel that it is at all questionable, then they will lock the account and look into it. During this time, people can still put money into the account, but you can't get to it.
This is the issue that people have with PayPal.
Prior to getting bought out by EBay, there were reports of people at PayPal stealing credit cards and using it on porn sites.
If you are using it for legit (legal) purposes and not with large amounts of money, and avoiding online casinos and porn sites, then I have never heard of a problem with it.