I'm a firm believer in the write it down strategy, so for the past ten years or so I've been carrying a Fisher Bullet Space Pen in my left front pants pocket. They're great pens and I recommend them highly. As you might guess, over the years I've lost a few, and at $20 or $30 each, that's always a bummer.
I have a couple of alternative pens that I sometimes carry if I think I might be engaging in activities where I could lose the Space Pen. Activities such as flying, shopping for clothes, or bar crawling. My alternatives are an inexpensive pocket pen I found at a Japanese stationery store, and the Cross Ion pen. The Ion really isn't much less expensive, and I dislike its gel ink, so I can't say that I recommend it. It's just that I've never managed to lose it, so I'll keep using it until I do. (Yes, I recognize the irony in the situation where I've never lost the pens that I bought for the purpose of misplacing them.)
I recently discovered a new alternative. It's the Fisher Stowaway Space Pen. It uses the same NASA-inspired, fantastic ink cartridges, but costs less than half as much as the more stylish Bullet model. It's a lot thinner than the Bullet, and about 1/4 inch longer, so it's still quite pocketable. It appears, however, that you can't replace the ink cartridge. I'll know for sure when the time comes to do so and I attack it with a pair of pliers.
At only $10, I no longer have to concern myself with my pen slipping out of my pocket. But no, you still can't borrow it.