In the United States, it is still used in the South and by older people, although it is used elsewhere as a joke or to give sentences a certain sound. I reckon that should answer your question.

It's common in the UK. On fraze.it, most of the example sentences come from British or Australian sources. "reckon" can simply mean "think" when giving an opinion, but in my experience, it's used particularly often when predicting or estimating something informally. "If he's not at home at this time of night, I reckon he'll be at the Black Horse drinking beer with his mates again." "Based on his recent form, I reckon Lewis Hamilton will win the next Formula 1 race."