A quick demo of how to get comments for a Blogger post with SimplePie.

gistfile1.php

<?php
$sp = new SimplePie('http://example.com', './cache');

foreach($sp->get_items() as $item) {
    echo '<h2>' . $item->get_title() . '</h2>';
    $link = $item->get_link(0, 'replies');
    if($link) {
        $comments = new SimplePie($link);
        foreach($comments->get_posts() as $comment) {
            echo 'Comment: ' . $comment->get_content();
        }
    }
}