As a relative newcomer to Joomla! (but learning fast!) and a complete novice with php, etc., it was a pleasure to be able to install and configure JComments on my 2.5.8 website in a very short space of time and with virtually no hassle. The only problem I had was my own fault: I forgot to save my SETTINGS preferences before browsing to another tab, and so had to configure them all over again (not a difficult job, actually). JComments is amazingly fully-featured for a free component, and so far has worked exceedingly well. It's unusually customisable as well.
Pros: Very easy to install via Extension Manager, loads of features, email notification to users of follow-up comments, quick editing by admins of comments via email, built in kCaptcha, flat or tree view, good documentation, nice price (free!).
Cons: None really. I do concur though with 'donnan' below that it would be nice to be able to change the heading 'Add new comment' to 'Add comment'.
Congrats and thanks to the developers!