Kindles are actually wicked light. The weigh less than most paperbacks. I carry mine around in my purse with a cover and I'm a weakling.
That being said, I LOVE my Kindle. I was hesitant about it because I like the feel and smell of books, but it is so incredibly worth it. I read a lot more often now because you can just take it out and read it whenever. Plus you don't have to worry about holding a book open (I know that sounds silly, but I hate when pages slip.) If I had to, I would buy mine again for twice the price I originally paid.