I actually purchased one of these and have had GREAT success engraving on slimlines. I'll get some pictures taken and posted of my results. Still learning though. I've found that freshly finished pens do NOT engrave well. The char from the engraving smudges and the finish gets tacky.
As for focusing the laser, I have had mixed results. I pretty much eye ball the precision. Text seems to engrave better than images. A wider pen, like a Sierra, has more surface to work with. I was surprised at how close I could get to the edges before the detail was lost.