Yes, as Angel keeps going they find their voice and the quality improves. But they also makes more demands on the viewer - trying more and more complex storytelling and going for lots of darker emotions. Series two and three are sort of middle ground for that - good quality but also lots of fun in amongst the brooding and gloominess. Series four is bleak and ambitious but somewhat sabatoged by the real life actions of one of the core stars. And series five you'll either love or hate - they break almost every genre convention they can think of, and try some of the most creative things I've seen (Angel as a Seasame Street style muppet for most of one episode, for example) - things you can only attempt in this kind of genre.