I have never used single rider at WDW (although I intend to on our upcoming trip later this month) but we have used it many times at DL. We find that it is very fast/useful at Radiator Springs Racers (cars and a track layout similar to Test Track) and Matterhorn, but not so useful on TSMM or Goofy's Sky School. Soarin', Splash and Indiana Jones are in the middle. The reason for this is all based on ride car configuration.
Matterhorn has cars similar to WDW's Space Mountain: two joined cars with 3 'single file' riders each. Radiator Springs Racers (like Test Track) has cars with 2 rows of 3 people. The majority of groups that 'go together' are 2 or 4 people. Imagine 4 people getting into a Test Track car: they will have 2 people in 1 row and 2 in another, leaving 2 single rider spaces. For Space Mountain, they will have 2 in the front car and 2 in the back car (particularly if 2 of these are children), again, leaving 2 single rider spaces. On Soarin', it is not uncommon to have a 'leftover' seat at the end of a row. Splash at DL has 'single file' seats, so again, single rider spaces often open up. Indiana Jones is 4 seats across (cars similar to Dinosaur) so while it is not as frequent, you will get groups of 2 or 3 that leave empty seats. But at something like Goofy's Sky School or TSMM, these are cars that hold 2 people sitting next to each other. You will need a group with an odd number of people before a single rider seat opens up there. Of course, it does happen, but not nearly as often.
So that's my 'single rider line' wisdom: if the car/seating arrangements are not conducive to groups of 2 or 4, many more single rider spaces will be available. For this reason, we definitely plan on doing single rider for Test Track!
The suggestion to have an adult go first is a good one. If you have 2 adults (or an adult and older child) have one adult/older child go first, with the younger kids in the middle, and an adult last. Then arrange to meet up afterward - we find that the ride photo area is a good location. This gives kids a bit of guidance and makes them feel more secure. It also allows the last adult to 'sweep' the exit area for any wandering children.
