1. In general yes.
2. Never dealt with them
3. In general yes, but lately they are very slow.
4. Not always. I'm quite disappointed of them lately.

A while ago I really liked poloniex. I also liked the margin trading they're offering and option to lend your money to make some profit. But I don't like how they block transferring out funds sometimes. I understand the necessity to do it sometimes, but when they do it for such a long time as they did it with the steem funds, they simply destroy their reputation. And that's bad not only for them, but for the whole blockchain.

I just published one post about this:
https://steemit.com/steem/@steem-buzz/poloniex-is-the-largest-owner-of-steem-after-steemit-50millions-usd-do-they-earn-any-interest-rate-do-they-have-any-incentive-to