I read them in publication order and i cannot imagine them being read any other way.
I have to agree. The first novel (I think) that I picked up from the library was 'Witches Abroad' , not realising it was part of a series. Have enjoyed it immensely , I went back to the first novel and read through in publication history.
The problem with not reading them in order is that you will be introduced to some of the characters 'mid-arc'; and whilst many change very little (eg The Patrician, Dibbler etc) others very much do (eg Vimes). You also run the risk of not seeing the gradual evolvement of Discworld in style, and so it may come as something of a shock to read (for example) Going Postal, then go back to Colour of Magic.
So I would say read them in order, and if you don't particular like his style of writing and references in the early books, stick with it. If you
do like it (like me) then so much the better, because it never goes away, it just evolves.
I do think there are a few of his stories that aren't that great, but I've found those to be some of later books like Unseen Academy, Monstrous Regiment and The Thief of Time.
The Witch, Guards and Death stories are all terrific, in particular the Guards which culminates in (for me) TP's best story which was Night Watch.