For speylines I personally don´t like them that much, at least the Airflo Polyleaders, which is great with shooting heads. With speylines I feel things get a bit clunky, presentation suffers etc because of the finer front tapers. In Scandinavia people are crazy about polyleaders, maybe because 95 % of the salmon anglers use shooting heads. A polyleader sort of smooths the action of these often short and heavy heads, kind of slows them down as well as, practically, making them longer, since a polyleader really is an extension of the fly ine (in my mind) becasue of the greater weight compared to a mono or flourocarbon leader.
Way off topic: For my spey lines I recently tried adding about 1 meter (3 ft 4 inches) of heavy flourocarbon to the butt of my flourocarbon leaders and that seems to work great with spey lines. The reason for this is that you can´t get any tapered flourocarbon leaders longer than 12 or 13 ft, at least not over here. WHY is that? The Loop LNL 17 ft mono tapered salmon leader is a great leader. I´d like to see a similar dimension flourocarbon leader on the market. The reason people in other fly fishing fields use flourocarbon seems to be becuse of the advantage of stealth. But that doesn´t matter much when fishing for salmon (in most cases). I like it for salmon because it turns over well, it´s tougher, it keeps your light weight small summer fly down even when fishing floaters in fast runs and it gives you better contact with the fly (and the odd fish

) than mono.