Phil Haack wrote a post about responding to feedback on your Open Source project yesterday. If you run a project you will either benefit by taking his words to heart or you already have (or you are probably very sad <smile/>). Even if you don't run a project, I encourage you to read his blog post then come back here for one follow up item.
Remember that cartoon where the Oatmeal character is saddened by the negative feedback. That's a human reaction to being told that what you've done is not good enough. It may be the truth but that doesn't make it hurt much less.
So, if you are going to tell someone what they do isn't good (or worse <smile/>) keep in mind that there is a human on the other side of your feedback.
In other words, don't take Phil's blog post as an excuse to be cruel when providing feedback. If at all possible try to provide constructive feedback. Maybe lead off your feedback with things that you appreciate then explain how the parts that don't work as well prevent the whole from being awesome.
Practice empathy. It's good for you and it's good for them.