Whichever version of the 1-Series you're looking at, the key descriptors of its interior are "attractive," "comfortable," and "easy to use." Laid out like 2+2s more than true four-seaters due to the small back seats, there's plenty of comfort up front for adults, with children the only real candidates for the rear seat.
A pass-through split-folding rear seat offers more cargo space for longer or larger items, while the trunk itself is fairly roomy, even in convertible models.
Overall build quality and materials are very good, particularly considering the 1-Series' place at the entry point in BMW's lineup, matching the 3-Series and 5-Series in most respects. Slightly lower-grade leather and plastics in some areas are the primary concessions. Cabin noise is low, with wind and road noise well-controlled, even in the convertible.
On-road comfort is good, depending on the performance options chosen; low-profile tires on large wheels can create more ride harshness, but even at the sportier end of the spectrum, the 1-Series is a controlled cruiser.