I like keeping a small pen in my pocket for moments such as when I'm at the ATM and realize I don't have anything to fill out the deposit slip with. I've used the Inka pen and PicoPen and found both to be very capable. However the Inka is slightly large for my keychain, and the PicoPen, while small enough, is a little uncomfortable to use because it is so small. The Telepen hits a nice sweet spot. It is actually the shortest of the 3 pens, although it has a larger diameter than the PicoPen. But once you extend it, it is just long enough for me to hold semi-comfortably. Because of the smaller-than-average diameter, it's not as comfortable as holding a regular size pen, but it is good enough to use for short periods of time.
One feature that is a plus and a minus is the cap. The pen snaps very securely into the cap so there is very little danger of it accidentally falling out. However it usually requires two hands and a very firm pull to remove the pen from the cap. I prefer the one-handed easy opening of the PicoPen.
The build quality seems ok. I have been EDCing it in my pocket for a couple months and it has held up fine so far. So it appears to be solid enough to withstand normal wear and tear even though it doesn't feel quite as well-built as the other pens.
It comes with 3 refills, but each refill contains less than 1" worth of ink so it doesn't hold much ink. But you can refill it easily since it uses the same diameter refill as some other pens such as the
BAZIC Silver Mini 4-Color Pen with Cushion Grip 4 Per Pack,Red/Green/Blue/Black, although you have to cut down the length to fit in the Telepen.
One other plus of the Telepen is the low cost. It's very inexpensive compared to most other keychain pens.
Overall I like the Telepen best for keychain duty. The PicoPen is a better wallet pen because of the smaller diameter. And the Inka Pen is more versatile because it can be extended to a full-size pen. But for everyday pocket duty, the Telepen works best for me.