I find that in most cases, concerns about compatibility of products written with 3.0 in mind for 3.5 is a BIT overblown. There are exceptions, of course. Classes which actually have character statistics (especially bards and rangers) really do need some tinkering to get 3.5 ready.
In some cases, you might find that new feats or modified rules are things that are fixed in 3.5.
One big gotcha. As little as I like the "Schools" in the path books, they are even more problematic in 3.5, where the FR experience rules become standard.
But anyways, I still use some Quint Books and some Path books in my game. But then, I don't totally run 3.5, so YMMV.