UPDATE:Using the plastic part attached to the back of the Rite in Rain ink cartridge makes the Fisher Space pen refill work the exact same. The dimensions of the cartridge are seemingly exactly the same.UPDATE:I had thought the clip might be too stiff and the pen too heavy to hold in the front shirt pocket but I haven't had any issues with either. It stores just fine in the shirt pocket.I have been looking for a heavy duty clicky pen for a while now but never found anything I liked until I saw this. The problem with the other pens for me was that they were either twist off caps which meant I wouldnt use them as often or they were just too bulky and looked too much like weapons. While this pen has an aggressive style it still looks like a pen and has the basic shape of one.It isn't too thick or bulky despite being a little heavy which is expected from the sturdy steel used in it. It is about average length for a standard sized pen and just a few mm thicker throughout unless you're comparing a sumo grip pen which is of course much thicker. With it's weight I wouldn't be putting this in my shirt pocket because it is substantial compared to other pens but the tradeoff is solid materials and solid construction.If feels very comfortable to write with regardless of the weight, however, you wouldn't catch me writing novels with it. The grip cutouts throughout the length of the pen do a good job providing grip while being cut well and not being sharp. I'm not all that familiar with the Rite in Rain ink cartridge but it writes very well and is very reminiscent of the Fisher space pen cartridges in writing ink flow and physical size and shape of the cartridge as it is also pressurized like the space pen. I haven't test whether or not the space pen refill will work yet but looking at the two it would seem a little modification at most would make it fit well.The spring in the click works well and feel heavier than a normal pen spring. It isn't particularly loud for and there is not a loud and specific click but there is a sound from the metal button sliding into and out of the pen when compressing it. The pocket clip is sturdy and stiff and could be a problem for your favorite jean pockets if not careful but it is simple and incorporated well into the design of the pens cutouts. Where the pen is sturdy and solid the clip matches.I won't speak to the "tactical" nature of the pen much but it does of course have a glassbreaker on the tip. It doesn't interfere with writing at all and I suppose it is nice to have it included. If you were to use this in a defensive situation, I can say it will hold up far better than any standard pen. The threads are strong, sturdy and go deep into the pen from the tip and from the rear. It would take a lot of effort for someone to break this pen.Overall I really like the pen and all of its features. I'd really like to see a titanium version to bring down the weight of the pen even if it drove the price up substantially. I'd also like an option to use a different tip without the glassbreaker. It would be horrible to end up in a situation where you need it and have this pen without the glassbreaker, but I would expect the sturdy tip to be able to help as well. Either way, I would have liked the option. If they release this exact pen in titanium I would pick it up without question. As it stands, it is a great pen and an easy recommendation from me and my initial thoughts. Should I see reason to update the review I will and am able to answer questions about it.