There is an excellent post from Google's John Mueller on the topic of user generated content and how important it is for that user generated content to be written properly. Forums and comments are known for having horrible spelling typos and horrible grammar issues. Will that impact your forum, blog, etc's ranking?
If users are creating content for your site (eg if you have a wiki or something similarly user-created), then I think it's definitely a good idea to help them to create high-quality content, be that by providing a spelling checking mechanism, or by making it possible for other users (or you :-)) to fix quality-issues as they are found.
On the other hand, if these are comments or testimonials left behind by users, then I think it would be a bit weird to modify them. Would you like to have that happen to feedback that you leave behind on other sites? Personally, that would bother me a bit. As the site-owner, I think it's fine (and usually expected) to make an editorial decision in whether or not a comment should remain on your site. Ultimately, it's up to you do make a decision on where you want to draw a line :-).
One way to think about this is to look at what users are searching for when they reach pages like that. Are they looking for the content in the comments? If so, one solution could be to take some of those comments and to keep a cleaner version within your own content, referencing the exact user comment further below on that page. For example: "Update: In response to this article, XZY left an insightful comment saying ABC. You can find the full comment below."
Another possible solution might be to allow users to "+1" (upvote) comments, and to only show the most insightful ones by default. Users would still be able to view all comments (maybe on a separate URL that isn't indexed, or through the use of AJAX), but by default, the ones shown and allowed to be indexed would be the ones that your users have found to be the most important ones. Depending on your audience, I imagine that could result in the lower-quality comments disappearing on their own.
Hope this gives you some ideas :) -- it would be great to hear back from you regarding your choice and how that works out for you.
That is advice from Google's JohnMu - a Googler I highly respect and trust.
Forum discussion at Google Webmaster Help.
Update: Google's official Google Twitter account tweeted this blog post.