I tried BofA's one a long time ago. From what I remember it was a weird Java based thing that you access from their online banking portal. It seemed to work okay the one time I generated a virtual number. I honestly didn't feel the need to use it for online purchases though (I mostly stick to "well known" sites like Amazon, or otherwise use PayPal), so I forgot about it.

Actually, I just logged in to took a look and it seems to be Flash-based. A bit better in that it's more likely to be on one's machine, but may cause problems on a mobile device. That may be okay though considering that a fair number of apps let you use e.g. Apple Pay to pay for stuff now.