The synthetic puffy insulation jacket is a cornerstone insulation piece. I have had one in my bag for almost every trip I have done in the hills, for almost the last 10 years. It is extremely versatile and useful. It is my choice compared to a fleece or down jacket in most circumstances. It is warmer for the same weight than a fleece, as well as wind and water resistant. A good down jacket can be warmer for the weight, but with a sacrifice of robustness and performance when wet. I originally used one as a 'belay jacket'. The idea is that when moving you generate a significant amount of heat and can dress lightly to prevent sweat/overheating. As soon as you stop moving, you will tend to chill, so you can throw the belay jacket on top of everything. It's pretty important this is a synthetic as it can absorb moisture from your clothing, for example if you have been snowy mixed/ice climbing and stop to belay, or sweaty from tramping hard uphill. It can be put on top of whatever clothing you already have be it t-shirt, softshell, windshirt or hardshell. This is very commonly done in Scotland, or mixed climbing in the Remarkables, when leaders can be very slow climbing difficult mixed pitches, but less commonly in alpine climbing when you tend to move faster. It is also useful in colder conditions to wear as an active jacket. I had a pulsar as my outer jacket most of the time on Denali, Alaska. It sheds some moisture and wind, but will not keep out a deluge, so I will wear under a shell in stormy conditions.
It is normally not as robust as a hard/softshell face fabric, so will get holes in if worm while mixed climbing or thrutching up chimneys. I will wear under a shell in such conditions, although I do not do this very commonly as it is too warm for being active. I often use the vest version over a Sonic windshirt, under a shell for extra insulation when mixed or ice climbing. When climbing the Supercanaleta on Cerro Fitzroy, Patagonia, It was much colder than expected and we ended up wearing our Pulsar jackets for most of the ascent. This wore a few holes in the light face fabric, but rather than in a down jacket, where you would lose insulation, the synthetic insulation holds together and keeps you warm till you get home and you can patch the holes. On our way up Aguja Standhardt a bag was dropped which contained my shell jacket. As we summited the weather closed and it began to rain for our entire descent which included 15 abseils. Without a hardshell my pulsar got pretty wet, but it kept my core top half warm and dry, even though my bottom half hot soaked through my softshell pants. This is not recommended, but the robustness of this jacket can be useful in bad situations like this.
I have used over 12 different synthetic insulation jackets from different manufacturers over the years, and the Macpac Pulsar is a standout for me. It is at the lighter end of what is available, with a lightweight Pertex Quantum face fabric. I like this as not only does it weight less in the bag, but it also makes it more useful as an active climbing jacket than a heavy, bulkier jacket. It can be packed into a pocket and clipped onto a harness for cold rock-climbing, or when you don't have a pack. I used the original Pulsar in Patagonia and Alaska, which was not specifically designed for climbing, but still worked well. The new Alpine Series version has been improved, with a hood that fits over a helmet, a longer body (needed when lifting arms) and a inside mesh pocket for gloves. It has good cuffs that seal well and work with a range of glove systems. It is great for most activities, and most seasons, I take it tramping, mountaineering or cragging in all but the warmest conditions.