Jay Neely has published a blog entry comparing Yahoo Pipes, Microsoft's Popfly and Google's Mashup Editor. I don't know anything about Google Mashup Editor because I haven't used it yet so I can't comment there but I've had a play with Pipes and Popfly.
Whilst it was an interesting read it confirmed an opinion that I already had - it is largely inappropriate to compare them all. To compare them side-by-side might cause the reader to infer that essentially they do the same thing, and that isn't the case. Jay himself says:
"Popfly is much more about images than it is information"
"Popfly doesn’t offer any true output function"
"[Yahoo Pipes is a] powerful tool that let users express ultimate control over how they used their feeds"
He's quite correct. Pipes and Popfly don't actually do the same thing. At a high-level sure, they take information from more than one place and munge it together but the presentation of that data is wholly different. We're not talking apples-to-apples here.
Still, its an interesting piece that illustrates where Yahoo, Microsoft and Google are going in this space. Its well worth a read if mashups are your thing.
-Jamie
Posted to Popfly (RSS feed), Mashups (RSS feed) & Yahoo Pipes (RSS feed)