I bought 2 of these through the Kickstarter campaign last year, and am pretty satisfied with the look and feel, the nib, everything but the o-ring arrangement for posting the cap.
The o-ring is much too easy to lose on the back end. The main section o-ring for closing the cap doesn't move, but the posting o-ring gets a little out of register, pieces of the o-ring sometimes get shaved off when trying to post, and in general, I find that to be a poor design choice and often makes posting awkward.
After loosing 2 o-rings, I am currently trying a drop of super glue under the o-ring to see if that improves the experience - it is still not always easy to post the cap...
Regarding the weight of the aluminum versus the brass, I definitely favor the weight of the brass version in my pocket for "every day carry", and it shares that honor with 3 Kaweco brass pocket pens (different models and sizes). Since the aluminum one feels a little too light, I keep it at home as a quick extra pen in my living room or at bedside that is always ready to write due to the o-ring cap seal.
I think this drop is good value for money overall, especially the brass model that is currently listed at $69 on the ensso site.
I've had the same thing happen with my pen. The o-rings are really prone to falling off. I need to get some extras for when I inevitably lose the last one.