The Z1 isn't Samsung's most popular phone by some distance, but that's because Samsung intends to have it that way. The phone was launched a month back in the Indian market, where it did well despite having more competition than comfort would allow for Samsung. To put that into perspective, Samsung sold more than 100,000 units of the phone in less than a month in the price-sensitive Indian market, and another 20,000 in Bangladesh, where the phone was launched only in the first week of February.
According to sources, the company, encouraged by the success that the Z1 saw, might already be working on a successor to the device. It'll most likely be known as the Z2, and from what we hear, it'll have slightly higher-end specifications, including a higher-resolution screen (presumably a 960 x 540 pixel panel), and more.