Whether I would recommend ShareTool depends heavily on what you’ll need it for, and whether you already have a MobileMe account. The Back to my Mac feature of MobileMe allows for File Sharing and Screen Sharing with relative ease, but ShareTool enables a host of additional services. Some, such as remote printing, can be achieved fairly easily through a combination of File and Screen Sharing, but others such as iTunes Sharing are only possible through ShareTool.
The price difference is important. MobileMe costs $99 for a year, whereas ShareTool is $30 on a one off basis. If you already own MobileMe and only require Screen/File Sharing, then ShareTool isn’t necessary. If you are not a MobileMe user, it’s a great way to stay connected to your home or office network from anywhere.