I was recently asked my opinion bout looped leader. As opposed to a regular leaders, that are attached to the float line directly, using a blood-knot, a small, looped tag line is fastened to the float that the leader, also having a looped end, is woven through.
There are a few advantages to a looped, such as the ease in which the leaders can be changed out. When changing out regular leaders, due to a hard knot or a high break, it is necessary to cut the old leader off, and retie a new one, thereby wasting a few inches of float line each time. With a looped leader, one can simply remove the old, and rethread the new.
While this is very handy, the visibility and drag of a loop is much higher than a small blood-knot. Due to those factors, I typically will only use a looped leader when fishing saltwater, or steel heads in rapid water, where visibility and drag are not so problematic, and for such situations, I find them very useful, however I would not recommend them for lighter trout fishing where it is preferable to to keep your line very streamline and clear. All that said, I highly recommend them for heavy fishing, but have yet to find a plus to using them for stream fishing.