Google provides more link data
I wrote this post a while back. The content can still be relevant but the information I've linked to may not be available.
Google is providing much more link data if you are signed up with Google Webmaster Tools. The amount of information is much greater than using the link:operator in normal Google searches (in my experience this was often a gross underestimate). The new system provides more information to anyone who has verified their site with Google. The date can also be filtered and downloaded (which will make it useful for sending to clients, ahem).
The information looks much more comprehensive in terms of numbers but it's important to say that how Google uses the information to rank your site is not revealed. That would be asking too much!