Pros:

• For the coins they have, you can't beat the volume, which means awesome prices (both buying and selling).
• Their API is top of the line in all the ways that matter (number of endpoints, documentation, data consistency and predictability, security, etc.)

Cons:

• Unreliable. I've seen the site completely go down 5 times in the past 5 months...I haven't seen any other exchange I use have that happen even once.
• Customer support is atrocious. I once had what seemed like an unexplained purchase and after 5 days they told me that it was impossible for me to have the problem I actually had and closed the ticket.
• Transfers disappearing...I like everyone else have started running into woes transferring coins in/out of Poloniex. I currently have 2 tickets open on transfer into Poloniex that have either vanished (ETC) or been stuck at 0 confirmations (FCT)...these problems are real, and it seems like a bigger and bigger percentage of their user base is running into these issues.
• Their number of coins continues to decrease. I think they've only added 1 new coin in the 5 months I've been using them (GNO) and they've delisted plenty of other ones with no transparency into why they list/delist coins
• They removed the trollbox. I would never use it, but its existence attracted users and gave people a place to vent and listen to each other, which I think had a positive effect on trading, the market, and Poloniex user experience as a whole. Removing it was a bad decision.

Overall:
All of the cons are risks, and they make me use Polo less and less...however if you are an active trader, it's super hard to ignore the volume and the prices of the coins on Polo, so I keep coming back but am using it less and less...if bittrex could get more volume, or bitfinex could get more coins, I'd be done with Poloniex for good...it's a necessary evil for me at this point @jerrybanfield