For those not familiar with lithium ion chemistry and the vent-with-flame potential I happily refer you to this Candlepower forum area: Battery Kabooms
It will educate you more than you probably ever wanted about batteries, their chemistry, their faults and benefits.
So why the change? Because I got a chance to check out a PD35 light at REI this weekend on DH's birthday, and having a 20% coupon there along with my dividend, it was remarkably inexpensive for a light capable of taking an 18650 single cell Li-Ion battery. The light typically retails for $80USD. DH liked it so much that he encouraged me to buy it--"Here, love, twist my arm to make me buy another flashlight!"
There are a number of nice things about this PD35 light. For a light capable of running on a single 18650 battery, it is very compact. It is not really any longer than DH's Nitecore MH1C 1xAA light. That's pretty impressive if you ask me, since an 18650 packs a lot more power into it, than a 14500 battery. It also has a small head/reflector on it, which means it fits comfortably in one's front pocket. This is nice, if you don't want to have it in a belt pouch. The pocket clip is removable, though I plan to leave it in place regardless, to use it as an indexing point to find the grey side switch. The notched fins on the head help prevent rolling when you set the light down, and also increase the heat dissipation when running the light on the higher output levels.
On to the switches on this light. There are two. This is less complicated than it sounds. The main switch, is on the rear of the light. It is a "tactical" switch, in that it has a momentary forward press as well as a forward click to turn on the light. The switch is large enough to find easily with a thumb. When pressed, the light will come on at whatever power setting it was used at last (except for strobe). So if you turn it off in Eco mode, it will turn back on, in Eco mode.
The steps in brightness in the main output settings are fairly well spaced. Even turbo is significantly brighter than High, especially when using an 18650 battery. Eco, which is the lowest mode, is fine for wandering around the house in the middle of the night to avoid stepping on sleeping dogs and kiddie caltrops. It's a bit too bright to truly preserve your night adapted vision--for that you would want a sub-one lumen output. Eco mode is about 10 lumens. There is available, a red filter, that fits this light if preserving night vision is important to you.
Because this is a 2xCR123 or 1x18650 light, it is long enough to use the Harries Method when carrying a pistol. My small 1xCR123 lights were really too small (and all twisty lights) to use in conjunction with a pistol. So why not use a pistol mounted light? Because I don't want to have to point a pistol at something just to put light on it. Think about that for a second...
Overall, I really like this light. It is very unobtrusive on the belt or in the pocket. The light comes with a pouch, a small lanyard, spare o-rings, a pocket clip, and a spare rubber tail cap. The cordura nylon belt pouch has a velcro flap and the belt loop on the back will handle a 1 1/2" wide belt. I don't know if it will handle a 1 3/4" belt but it might. If you register your light with Fenix online, they will extend the 2 year warranty an additional 6 months, which is nice.
For those of you who want to see beam shots at night, and some white wall hunting pics, I will gladly refer you to FlashLion's review on CPF. It is a very detailed review and includes some output measurements in a real-world setting. He's also a better photographer than I am with my cell phone!