Lenspen has been around for a few years now, but this weekend was the first time that I had a chance to try one. Just to get one thing out of the way, yes it does work as advertised. The tool itself is small and easy to use, which makes it easy to get into the corners of lenses where you might have trouble with an ordinary microfiber cloth.
The company touts that their cleaning products use carbon to absorb oils and grit, much like how people used to use newspapers to clean windows, printing ink containing carbon black back in the day. This is pretty much the same thing as the activated charcoal in your aquarium filter; the carbon molecules are extremely porous, and pack a lot of surface area per volume.
You would think that carbon would be too abrasive to use on a DSLR lens coating, but the company cites endorsements from the major camera manufacturers such as Nikon, Olympus and Leica. The product has too ends, a fine-bristle brush to knock of major dirt particles, and the actual cleaning end which looks like a black felt pad. The actual cleaning process doesn't require much pressure, and for the most part, it does pick up oils with a few swipes.
A common complain with microfiber cloths is that they can smear the oil around. This isn't a problem if you always remember to start off with a fresh section of the cloth. Usually, huffing on the lens before wiping helps the cloth clean better, but if for whatever reason you've got an extra amount of gunk on the lens, there's always cleaning fluid.
Would I buy it? Maybe. As with many of the things that I've used, it depends on the price. Online prices hover around $5 USD with B&H and Amazon et al, but these things can be pricey if you pick them up in a retail location. As I mentioned, it does a good job of getting into the corners and edges of things; but I am worried about the small size of the cleaning surface.
Every time you re-cap the Lenspen, the carbon gets replenished. However, its not the longevity that worries me so much as it is the actual usage. When you use an ordinary microfiber cloth, you tend to use in one area and then move on to a fresh part of the cloth for the next time. When you are cleaning with a small surface like the Lenspen, its no problem if the surface gets exhausted and gummed with oil, but if it picks up grit and any other particles, you are still using the same surface. For me, that's a theoretical concern, and having sifted through a number of user reviews, for the most part, people have had a positive experience with this cleaner.