The value there, IMO, is more the links than the discussion. It's not so much like HN, where the discussion itself is half (or more) of the value. But, for a quick, concise list of new links in those fields, I find those Reddits all very valuable.

better choices are http://metaoptimize.com/qa http://blog.kaggle.com/category/how-i-did-it/ and following ml topics on quora

Yes, those are excellent sites as well. There are also some good StackExchange sites that can help one get their fill of CS'y topics.