1. When you set the attack really fast it can make the sound less open, because it lowers and affects the transient.
2. With regards to the release; faster is better until it sounds bad. You'll notice if you set the release to the fastest and are doing a lot of compression at high ratio as you let chords ring out it sounds really un-natural. Also, you'll notice with some strumming patterns it can get a bit distorted almost with too fast of release. So, with the release, start it at its fastest, and then slow it down until your sustains sound natural.
3. A great way to get a feel for compressors is at extreme settings. When you're experimenting with the attack and release and ratio, don't be afraid to set the input so you get 15-20dB of gain reduction. It'll make it easier for you to hear things.