Should I use a DNSBL to check web traffic to block spammers?

Rather than solely fighting off spam with CAPTCHAs and spam comment checkers - is it a good idea to check each request against a DNSBL and block the user if they are using a bad IP?

$blacklists = array('web.sorbs.net', 'opm.tornevall.org');
$parts  = explode('.', $_SERVER['REMOTE_ADDR']);
$ip  = implode('.', array_reverse($parts)) . '.';
foreach($blacklists as $bl)
{
    $check = $ip . $bl;
    if ($check != gethostbyname($check))
    {
        error_log('PHP Security: [DNSBL] - ' . $_SERVER['REMOTE_ADDR'] . ' - ' . $bl);
        die('Put a detailed error here so the client knows why they have been blocked');
    }
}

It seems like the only problems would be over-zealous IP blocking of good users by the DNSBL or the large cost of making a DNS lookup each request.

This might help, but you will have to take two things into consideration: False Positives and False Negatives.

DNSBLs tend to have quite a few of both. False Positives that hit innocent users, and False Negatives that will miss good chunks of botnets. The best solution I have found for dealing with spam online is to use CAPTCHAs.