I've switched to coco coir in almost all of my outdoor containers last year, and really liked it. Haven't tried it for seed-starting yet, but probably will next season. It's awesome for self-watering containers (I do a lot of growing in Earth Boxes due to my limited space/poor soil). It does need a regular and consistant fertilizer, though. Even through our horrible rainy summer last year, I didn't find overwatering to be a problem.
Coir is so loose, it drains quite well in my opinion.
Everyone's situation is so different--there is no "one size fits all" solution, unfortunately. It's nice to hear the pros/cons of different methods.
