Mine came in earlier in the week. The rear o-ring snapped when I removed the tail cap and it doesn't appear to have been lubed, the front one did the same. No big deal, I can replace them easily enough. I cleaned up the trademark DX machining swarf, popped in a battery, and WOW is this thing bright!

The drop-in module seems just as bright to my uncalibrated eyes, but with the all-important flash mode. For what it's worth, I have two other strobe-mode flashlights from DX, the Ultrafire U4-MCU and the Ultrafire C1 R2-WC. The drop-in module's strobe rate is a tiny bit faster than the U4-MCU, but quite a bit slower than the R2-WC. It's pretty attention getting, but I'd be thrilled if it had a slow strobe rate option.

Now for the bad: the light stopped working within an hour of use, as I was tinkering with the two LED modules. It turns out that the tail switch went bad within a few dozen cycles. Again, not a major issue since I've decided to mount the drop-in module in a custom housing. This is the first problem I've had with a DX order, but I understand that the quality control is spotty.

In the end, it's my opinion that the WF-501b is a decent light and an amazing bargain at $9.90, but the XP-C-R2 is an even better starting point for a monster DIY tail light. Hope this info is helpful.