Smallville was and still is my all-time favorite show. I started watching it in high school, borrowing my sister's DVDs to catch up on earlier seasons, and it was the first thing to make me fall in love with superheroes. I have watched the entire series about 5 times since then.
Seasons 1-4 could basically be watched as stand-alone episodes without needing much of any background to follow what's happening. Each episode in seasons 5-10 build on information brought to light in the previous episode, so it's important to watch them in order.
While the graphics at the beginning of the series are fairly cheesy, they quickly improve throughout the series. I highly recommend Smallville to anyone who has even the slightest interest in superheroes.