It is a good subject to talk about and share our experince ..
So far I have used Poloniex.com , Cryptopia.co.nz , localbitcoin.com , poswallet.com ( but it is changing its changer status ) and bartermyfund.com ( it is very straightforward )
I have never had any problem with them. I would recommend all.
I started using Poloniex recently, but had a bad experience. Had a withdrawal that never made it to the wallet, it took forever for them to respond to me (my coins did come a day later). I probably won't use them again, but besides that, I guess they were OK.
The Coinbase site doesn't work for me all the time

I preferred not having to submit so much info to Coinbase, Poloniex was so much faster to start using.