I tried one of these units out at a recent Rockler woodshow near here (Minneapolis). They are a *great* leap forward for airless nailers. I almost bought one until I reminded myself that with the same amount of money I could buy a small pancake and nailer.... The cost efficiency goes down further if you need a 15 gauge as well as an 18 gauge.The air free units work by spinning up a small flywheel that then discharges its kinetic energy into a pin. This is different from the previous electric nailers that didn't have that extra mass (and thus weren't powerful enough for 2" brads), or from the other airless units like the older Porter-Cable that used chemical cartridges (that caused jams or ran out unexpectedly), or the newer P-C ones that have a small compressor on them (that are heavy).
The Sencos put a 2" brad right through the 2x4 pine they had at the show. You can control the depth of shot and the trigger action is very smooth. There are safeties to make it harder to put a brad through your thumb but if you want to do it, its always possible with a little due diligence on your part.
There was very little "spin-up" time and while you can't go bam-bam-bam-bam as with a pneumatic, you can go at a pretty comfortable pace. Not having a cord would be great for working on a ladder or in a location where electricity hadn't been pulled in yet.
The batteries seem pretty solid. I was at the show all day and it didn't look like they had to replace the batteries once. I wish the batteries were compatible with Porter-Cable or Dewalt or some other series so that you could share chargers. I'm sick of the pile of battery chargers I have in my shop.
Overall, two thumbs up. If they can get the weight and price down, that would be a further bonus. But you're buying from one of the leaders in high quality nailers, so the price is eminently justifiable.
