It flows decently, but not quite as well.
Using Naruto as an example, the filler arcs have a better leadup to the timeskip, and it develops some characters, like Hinata and Gaara. Shippuden's fillers also develop the likes of Aoba and Okisuke, plus it introduces Utakata before the World War.
I haven't finished Bleach yet, but the Bounts are brought up again later as part of Ichigo's character development.
Some of One Piece's filler is material that Oda couldn't put into the manga, like how Usopp got his goggles.