Scott Bolinger talks about what was learned after professional user testing was conducted on the new WooCommerce Smart Checkout product. Testing was done with usertesting.com. In the end, Scott definitely recommends it.
I know it's not on every client (or in-house budget) but Scott says: “You may not need to use a paid service, you could ask some friends or co-workers to help. The problem with asking friends or co-workers is that they are not a diverse sample.”