I thought the last DLC (Knife of Dunawll) was pretty amazing. It has good length of gameplay considering its price, and the overall quality was top notch, unlike typical single player DLC with filler side quest-ish contents. The new levels were structurally well thought out, like the ones in original singleplayer, were beautiful and added depths into the unique world they created. The new characters were great and the narrative was tight.
To be honest, I can't think of a better Singleplayer DLC for a FPS like this.
As for the release schedule, yeah I think it would be great if they release it much sooner, but better late than never.