Boulder pads are climbers' best friends. They stop us breaking our ankles coming off boulder problems, give us somewhere to wipe the mud off our shoes before hopping onto the rock, and last but not least, provide a comfy place to have lunch. In fact, a good pad propped against a handy boulder can be a little too comfortable and encourage napping rather than hardcore bouldering. There are a number of things to consider when buying bouldering pads. The first is size. A generous pad will make safe landings much easier. It'll give you more chance to cover uneven ground in the fall zone and mean that your spotters will be able to concentrate on what you're doing rather than shifting the pad to stay underneath you. On the other hand, the bigger a pad is, the more difficult it is get it to the boulder field in the first place.
Third on the list, there are the design details- the carry system and the outer material. A backpack-style carry is much easier for long or steep walk-ins but side handles are useful at either end of the trip. The casing should be tough enough to deal with sharp rocks and rough handling, and some pads also have a slightly rubberised outer for better grip on sloping surfaces. A boulder pad made from high quality foam will be much lighter than a cheaper equivalent, and it'll also last longer. Black Diamond, for example, make products with a pretty good size, durability, and cushioning power for a surprisingly moderate weight.