There are no major problems to be wary of the with Daihatsu Sirion. It's based on the Toyota Yaris (Toyota owns Daihatsu) and as a result is good to drive, well built and reliable. The reason they tend to be so affordable is that Daihatsu doesn't have the same brand image as other makes and the Sirion isn't well known. But it's a great small car and superb value for money. Daihatsu dealers are also (usually) very good with excellent customer service.