It's not as good as the real thing, but it's great for practice. It can help you with being consistent at many levels - on/off throttle, braking, driving line, passing etiquette, driving long mains, etc. It's also nice being able to drive all those different tracks around the world. If you think your driving needs some work, I think it would be helpful.
Quite a bit of feedback in these threads as well:
http://www.rctech.net/forum/showthread.php?t=133692
http://www.rctech.net/forum/showthread.php?t=60207