My attempt to explain why 650W units are usually bad. First, in many cases they lack the power for higher end CF/SLI. If you want to SLI the 1080, that's 400W for just the cards. Add in a CPU and system and you are looking at 525W+. This is already close to 80%, and if the cards need more than 200W each or you want to OC things you might not have the plugs needed to run them, or you'll be pushing the PSU over 80% output which most (including me.) don't suggest. Second, from the links nerd389 provided, there is a $15 difference between the 550 and the 650W unit. ($90 vs 105.) This is where the math starts to come in and you need to look at efficiency.
At idle or watching netflix/surfing the normal power draw for just about all systems is very low, 80-100W. If you have a 500W PSU then you know you'll be using ~20%, so you also know you have to be getting 82%+ efficiency. This isn't true with a 650W PSU. 100W is <20%, and unless you have a titanium PSU there are no 80, 82, etc percent requirements. This means not only do you spend an extra $15 at the purchase of the PSU, you will possibly be spending more money than needed while doing most tasks unless you can get the idle draw above 20%.
If a 650W can provide the power you need then great. But from what I've seen either a 500-550 for a single GPU is whats needed, or a 750W for a CF/SLI setup. If you are running some odd/crazy setup, or shooting for larger than normal OCs then I can see why a 650 might be needed. But honestly for most people the 650W units are odd ducks that get no love from me.