If someone ever asks you what you think about something, don't wussy foot around your true opinion. "That was cool" means just about as much as saying "hey -- you just did something." Feedback is something that means more than the saying a bunch of words to make someone feel better about themselves. While I never really want people to bash me or anything that I created, but I also don't want people to dance around their true opinion like they're going to get stabbed in the foot if they stop to say what they really think. Honesty should never be plan B.
Now, while I believe that honesty should be the only approach, I totally understand why it is hard to rely on. When seemingly cornered by the dreaded question, "what do you think," it is difficult not to notice the drooping eyes and slightly hushed tone of someone who clearly has done enough work to get angry if you don't fall in love with whatever they did: no it's not fair, but I'm sure I'm not the only one who has fallen into that trap. If you tell them you love it when you really don't, you're hurting their chances of making something the best it can be. Similarly, if you tell them that you notice the faults in their possible masterpiece, there's a possibility for some words and maybe more tangible objects to get thrown at you. It is a completely understandable fear to get mauled, verbally and/or physically by a droopy eyed person so throwing a half-hearted compliment to dodge them seems easy. Yet, if you decide to take this avenue, you have to remember what could happen as a result.
While it isn't malicious to try and preserve someone's self-esteem and confidence you could still end up hurting them when you dodge honesty. A possible confrontation is frightening, but it is necessary to remember that when a person reaches out for your thoughts, they may not know the quality that their work really is. After working for a long time, they have spent too much time with the same thing to determine if it is actually good or not: just look at those last minute term papers. The ultimate goal during intensive work sessions really is to get it done. Beyond that, the hours spent and effort utilized can blind someone from seeing how good or bad their work is. By reaching out to you in turn is to put heir trust into your fresh thoughts and mind in order to rejuvenate energy to revise and be better. Use honesty and take the hit.
I don't really mean that literally; I'd hope that no one would hit someone just because they dislike something that you did. Yet, I do believe that once someone reaches out for your thoughts you really can't think about anything other than making sure that they can do the best work that they can. They are trusting your opinion to make whatever they are working on better. Do not rob them of that.