I love Stardock, but I've been downloading stuff from them (GalCiv 2: Twilight of the Arnor and multiple patches/extras involved in that game and the other expansions) since before there even was an Impulse, and can confirm that whether you download through Stardock's earlier app directly (you still can, on some games at least) or through Impulse, errors, unexplained stoppages, and slowdowns are extremely common.
Additionally, Impulse seems buggier than the old program in one way at least. I have Sins of a Solar Empire, and I have not been able to download the latest patch. It's been out a while and I have tried many times. It starts up and pretty quickly stops. When their old app would do that, at least none of my screens would say that I was up to date. Not so with Impulse.
I downloaded Dawn of War: Dark Crusade last night. Quite slow. But Impulse at least wasn't buggy this time. Tell the truth, most of the downloads I've had with Stardock's earlier app or with Impulse haven't gone through the first time.
On another note, I am no fan of the strong stance Stardock takes publicly about not having DRM, but then allowing games on its distribution system with DRM -- but without letting anyone know about the DRM. It is quite misleading to come to one of the most publicized makers and distributors of DRM-free games to find yourself downloading a game with DRM. Especially since a download = an install automatically. If you find out a game has DRM on it, who knows if you can do anything about it once the download begins. Pretty lame.
All that said, I still like Stardock a lot and will continue to support them and use Impulse (very selectively from now on). To tell the truth, I was very iffy about buying Sins of a Solar Empire and would never have bought it if it were not sold by a company I explicitly wanted to support. I wouldn't be surprised at all if a great many people did the same.