I use the one that comes with DJ Event Planner (same as DJ Intelligence but better and more customisable). I have the checker on the home page as well as the disco, karaoke and race nights part of my site. i find it quite handy but clients find it even more handy as rather than phone or email to see if we have that date free they can check... So from an admin point of view you only get enquiries through your site that you are able to do.
And the good thing about it being part of DJ Event Planner is clients can check if i am free then send the enquiry with all the details of the event and when i log in i can double check the date, add the event into the diary and send out a quote within a couple of clicks...
If you haven't looked at DJ Event Planner i'd highly recommend it... As you can manage your diary, contacts, forms, requests - pretty much all the admin side of running a disco business...