I've been using this app for some versions now and it works exactly as advertised. I need to sync my calendars on two Macs and it does this without hassle and fuss.

Yes, so I've had to pay an upgrade fee once and yes, it is somewhat highly priced considering it's a one trick pony. And yes, it is not slick, shiny and pretty. BUT - you are entirely free to use .Mac.
However, If all you need is calendar syncing, this is cheap in comparison.

Plus. I've tried .Mac and I must say, I've had more trouble syncing and re-syncing and clearing up conflicts using that pricey service than I've ever had using iSynCal.