Here are a few of my issues with it:

• It crashed at least once.
• It feels quite slow, certainly slower than the Windows Remote Desktop, for example. Maybe because it's based on VNC and sending around pixmaps all the time?
• The clipboard is not automatically shared. You have to send it around explicitly. This means that you can't just copy and paste a URL around without going to a menu inbetween (or click on an icon).
• I restarted my wireless router with a screen sharing session open. The session just hung and was not able to pick up the connection again. I had to restart the screen sharing session.
• The "New..." connection menu entry just opens a dialog with a single text field. There is no way to browse from a visible computer on the network. Annoying if you just had to close the connection (see aforementioned problem) and just want to reopen it. You then have to go back to the finder.
• If you show the toolbar (with three miserable icons), you have a "Fit screen in window" option, which it turns out is the same a the "Turn Scaling On/Off" menu entry. Mmmh, did anybody at Apple even looked at this app?

I wouldn't say Screen Sharing is useless, but this is a version 1.0 and it shows.