I managed to get the 3.5 boxed set for around 3 dollars, so I upgraded.
However, I don't take them with me to games. It's amazing how much wear and tear you can put on books just by transporting them. So until I fashion some sort of armor for my boxed set, I won't be taking it anywhere.
Not only that, but the people in my group can be real monkeys when it comes to knowing how to treat other people's things. They do stuff with other people's books that I couldn't imagine doing.
So I don't let other people look at my books, not for a second; which I get a bad rap for, but hey, 1) I'm not made of money and these books ain't cheap 2) I am extremely particular about keeping my books in perfect condition.
Our regular DM's younger brother is running a campaign now and doesn't have his own books. He asked to look at my PHB, and I relented (for once). Not two minutes later he spilled his pop and some of it splashed onto my book. Yay, sticky!
You can promise to be careful with a book, but NO ONE can promise that "Nothing will happen", unless they're God or Neo or some crap; in that case, they can promise away.
So, basically I use the 3.5 books as reference material when I'm at home and take the 3.0 books with me to other people's houses.