It has "HD" in the name, and everyone knows that if it has "HD" in the name it's not only more advanced than anything without such a moniker, but also more expensive. It's a wonder, then, that miCal HD only costs $2.99, because on top of the fact that it's in HD (like every iPad app) it is one of the more versatile calendar apps available to use for the device, and it supports industry standard features like Google sync, Facebook sync, Outlook sync, etc etc.
Another really cool feature is that you can add GPS markers for certain events so you never forget where to go. Say that your office is having a celebratory party, but you just know you'll forget the address. Simply type it in when you're making the date on the Calendar, then when the day comes it'll automatically give you directions to where you need to get when you need to get there.
There are some complaints from a lot of users, however, about the app crashing, and while it's certainly not an all-encompassing issue for every user of the device, it's something to keep in mind before you pay for it. In the other hand, it's only three dollars, so worst come to worst you're not out much cash.