How old is your preschooler? Have you been to any drop-in soccer classes before? If you have not, I suggest you try it out first. If your toddler likes it and you have time (and money), then go for it. We tried it at 2 and 2.5 and DS was not into it. He is perfectly happy kicking the ball at home, but he was not into following instructions and doing all kinds of "drills." I have seen other 2.5yos, who liked the classes much more than he did. So just like with everything else, it depends on the child.
If you do decide to go for it, ask to meet the coach and also ask if it will be the same coach every time.