I have personally been using Coinjar to buy and sell bitcoins for the last 6 months. I dont use is as an online wallet, and if you do you should only keep a small amount on it.
That being said, buying and selling on coinjar is very easy, and I have never waited more than 48 hours for a deposit to arrive from a sale, or for my money to be credited to my coinjar.
As soon as you have bought BTC move them to an offline wallet. Simple.