Personally I like timing sets. Measuring exercises by time-under-load as opposed to the traditional approach of counting repetitions has a number of advantages. First of all you are less inclined to stop an exercise when you reach a predetermined number of reps. Secondly you are less likely to rush repetitions in the quest for higher scores. It is much easier to score higher rep totals by rapidly, bouncing through exercises using little muscular effort and a lot of momentum but this is not a productive use of your time and heightens the risk of injury.
However, the important thing is progress. Whether you monitor this progress by timing sets or counting reps is of secondary importance. Either measure can be gamed so choose one which works for you. Progress is the key, not how you measure the progress.