Window feeders are very popular in Ireland, but unlike other anglers, Phil and I never started a match using them. Instead we found it better to use them later in the match when bites were starting to slow.
Switching to a window feeder then would often produce an immediate response, and I think the reason for this was that I was casting a bit further with a window feeder.
Window feeders cast a lot better than a normal open-end feeder. This has the effect of making the braid between rod-tip and feeder a bit straighter, which obviously means I am fishing just past my main feed. A lot of fish will sit just off the far side of the feed as the match progresses, and a window feeder is a great way of picking them off later on.
The other benefit of a window feeder is that it doesn’t release much bait as it falls through the water. So if the swim is deep and you want to keep the fish on the bottom, try a window feeder!