No, at least for android there are r/w ones. None ive tried been perfect so far though

I'm iOS so I can't say anything about Android. All decent apps for iOS are read only.

Use mine - I think it's the best HN app out there:

https://itunes.apple.com/us/app/news-yc/id592893508?mt=8

It lets you login, vote, post and comment. The only thing it doesn't do yet (but updates will be coming soon) is voting in polls. I use it when I'm at home and I can get on my computer because I honestly think it's a better experience than using the browser.

No news:yc allows you to post comments. It's available for iOS. Not sure about Android. Using it right now; works really well.

So far the only thing I've used that I like, even without commenting is Premii's Hacker News.