It's come up a couple of times recently on local blogs, so just to make things clear:
"Erstwhile" means former -- your "erstwhile friend" is no longer your friend. I think people don't know what it means, but figure it sounds a bit like "worthwhile" so it probably means something to do with being worthy or respectable or something. Not so.
Bonus tip: If you do want an adjective that means "worthy of respect" with a touch of "formidable", go with "redoubtable" -- a personal favourite of mine.