When answering a question and a OP has further follow up questions to your answer. Is it appropriate to answer then using the comments section or how should further discussion be extended. This is especially important to users who have less than 20 reputation points and thus can't use the chat features.
Providing extended help in comments—especially to answer follow-up questions—is never appropriate.
Stack Exchange is not extended technical support: it works on the premise that specific questions beget definitive answers. If a user asks follow-up questions, they should be politely directed to do one of the following:
This is one of those "it depends" sort of questions, so it gets an "it depends" sort of answer. For me, this breaks down into three main categories.
The biggest problems with comments is when they're used for scope creep (e.g. changing significant details that are not folded back into the question), or represent follow-ons that really deserve to be separate questions. Comments requesting full tutorials are pretty common, too, but they are much easier to filter out.
I think judicious use of comments can be very constructive. In the end, though, the questions and answers should stand on their own merits without a lot of side commentary.