After receiving the InnerVibe Duet cock ring in the mail, I opened the tear-off package which contained just the ring. There's no manual that comes along with it, just some simple instructions printed on the reverse side of the packaging.

The Duet is composed of non-toxic, phthalates-free TPR Silicone (specifically, Opulux); hence, it has those stretchy, gel-like properties common to the material (also, being silicone, only use water based lube with this product). The whole product is waterproof, and can be cleaned using warm water and soap.

The ring carries two small bullets placed on opposite ends of the ring, each of which has its own batteries and on/off button (just push it in once with a small amount of force, and it will turn on/off). These switches were pretty easy to de/activate, even with 'lube-finger', which was appreciated. The bullets stayed in place extremely well. Another positive feature is that the bullets can be removed easily. The benefit to this is, unfortunately, heavily undercut by the fact that the batteries are wholly enclosed by the bullets; hence, no replacing dead batteries.

I did a few measurements (which can be found at the end of this) for anyone interested in using a different bullet with this. Basically, the included bullets have a 1.5" circumference. I was able to get my middle finger in it, which, after measuring, has 2.0" circumference. A bit bigger might be okay, but it was getting pretty stretched out at 2".

One side of the gel-part surrounding each bullet has eight little 'nubs' on it, and they form a somewhat squashed, elliptical pattern. If you lay the ring down on one side, bullet 1's nubs will be facing up while bullet 2's non-nub side will be showing; and if you flip it over, bullet 1's bare side will be showing while bullet 2's nubs are facing up. The tear-off package's instructions recommend wearing the ring so that the bullet, being worn on the top side of the penis, has it nubs facing outwards (for her pleasure). Also the instructions as well direct you, if using a condom, to put it on first (although I have no idea how this ordering could matter).

I got two uses out of this ring, (the second is discussed in the 'experience' section below). Using it by myself the first time, after water-based lubing up, I began by putting the ring just around my shaft and was surprised that it was actually quite comfortable. But with this comfort comes the trade-off of not having the ring provide as hard an erection as other rings. I then turned both of the vibes on. The strength of the vibes was also good: I could definitely feel them, but they weren't overbearing at all (nor noisy, for the record). After a few minutes, I wanted to see if the ring could go around my balls too, so I stretched the ring out and slid it over. This was very easy to do since the ring's maximum stretched diameter is 3" (note: the un-stretched diameter is 1.125"), again surprisingly comfortable.

Here occurred the only real problem I experienced with the Duet. Within five minutes, one of the vibes died. Seriously? As noted above, here's why this sucked: you can't replace the batteries, as the bullets completely enclose them (however, the bullets are removable so I suppose you could find some others to use with the ring). Given how I was wearing the Duet, the one that died was the one below my testicles, and so I flipped the ring around so that the still-working vibe could stimulate my balls and perineum a bit while I finished (having the vibrations on top of my penis doesn't do much for me really anyways). On a final note, the other battery died during my third use of the ring (somewhere around on hour or so of use).

Measurements
These aren't super precise, as I was using a standard ruler, so they might be a few millimeters off.

The Bullet:
Length - 38mm or 1.5 inches
Diameter - 12mm or 0.5 inches
Circumference - 38mm or 1.5 inches

The Ring's Bullet Holder (unstretched):
Length - 36mm or 1.4 inches
Diameter - 11mm or 0.45 inches
Circumference - 34mm or 1.35 inches

I was able to get it around my middle finger though, and measured my finger with a string: Circumference - 52mm or 2 inches

A couple days after my alone time with the Duet, I used the ring with a girl I'm with. With the Duet now (ironically) singing solo, I opted to wear it with the working bullet on top of my shaft so that she could feel it against her clitoris (with the nubs facing out, to be clear). This worked well in two positions: first, while I was on top and, second, while she was on top facing me.

Here's one thing she enjoyed while I was on top. I would push myself all the way inside of her, and then begin swiveling my hips in a circular motion while maintaining my full presence inside of her. Doing this provided two points of stimulation on her clit. One from my cock when I would reach the height of this circular motion, thus applying pressure on her sensitive parts (and slowing down a bit at this point in the motion helped, too). The other was from the ring's bullet and vibrations being moved in a similar, small circular motion against her clitoral area.

The other thing she enjoyed was, while on top, she was being able to sit all the way down on me and then begin grinding back and forth against the bullet a bit. Although she afterward said that, in either position, the nubs weren't doing much for her either.