Unfortunately, Availcheck, the free version anyway, will block calendar access after 78 hits per month which wasn't that cool really because out of the hits we get every day, the calendar seems to be one of the popular places to check out.
I then switched to Google Calendars and I must say that they are really good. However, one very interesting point that is worth a mention is the searchability.
I have noticed recently that I have had hits through the search engines direct to the calendar rather than through the site which then send the traffic to my site.
There is plenty of facility to add text and descriptions with links too and this could be a valuable tool in driving potential clients to your booking area.