I initially had some problems with this tool (I was an early user of it). The support was good and changes and fixes were made.
It's a good calendar tool, though it has some display issues where the available room for it is small (it does not display well in a 600px space - there are overlaps). I had a 1000px wide site with 2 200px sidebars and it couldn't handle it. However, I selected not to display the right side bar and it displays well at 800px.
Support is generally good, though I have experienced a few unanswered emails. But a big kudos to them, when I complained about it Jtag really got onto it and made it right.
For it's purpose, this could become #1 with the addition of a couple of items - the most pressing, IMHO, is emailing a user when they register to confirm details and sending a copy to the admin.