Don't get me wrong: Guadalajara is far from the first city I'd recommend anyone spending time in. Most of it is kind of ugly, the traffic is bad, and... well... I just didn't really love it. Having said that, the Old Downtown is quite beautiful, its churches and government buildings impressive and imposing and a bit European-looking. Oh, and did I mention the thousands of stores selling cheap gold and silver jewelry? :)
In short, don't make Guadalajara your final destination, but by all means do stop there for a night or two and spend time wandering the old streets.