He definitely grew significantly as a writer over time, and I would agree that some of his early work isn't particularly strong (The Light Fantastic, for example, is relatively bog-standard comic fantasy without any of the depth his later work showed).

If you start reading at the very beginning of the Discworld, you're slogging through the weaker stuff, and it's easy to get discouraged. A smoother path is to pick one of the defined sub-series (the guards are very popular, but my vote goes to the witches) and start along just that track; you'll get to the strong stuff much faster.