What is the most effective way of slowing down footage?
Depending on how much you are looking to slow down.  Interpret as 50fps, and then once in your timeline, say it is a 25fps timeline.  Then you can stretch the footage using time remapping, then you can turn on frame blending to the solid line...this will be good in most instances up to about 25% speed of original...

Less than that the same, 25fps timeline, interpret footage at 50fps, then use twixter to get that super slomo feel.  Not that twixter does not always work and is going to give varying results based on the footage.