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:
If they have a new problem—even if it's tangentially related to the problem of the current question—they should ask a new question.
If their problem hasn't changed (or evolved), but isn't solved by any of the existing answers such that they want to clarify their existing question, they should edit the question with the new information so people can update their answers accordingly.
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.
If their comments show a way that my answer can be clarified or improved, then I just make an edit and call it good.
If their question is slightly afield of the original question or answer, and the answer can stand on its own without an update, then I'll go one round of comments if it seems useful for posterity.
If the comments are too far afield then I either suggest asking a new question or politely ignore it.
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.