I agree with sending a PM if advice wasn't asked for. IMHO
If it's a "show and tell" they are proud of what they have accomplished. Be nice or be quiet...
If it's "tell me what you think" then comments to me are still on the nice side.
If it's "oh man where/how did I screw this up--I need help" then they have specifically asked for all the help they can get. Still be nice but offer help.
Never say something that could embarrass the maker...we want to encourage everyone to keep on keeping on :)