I have 2 alien posts.
5 years ago or whenever they were released (carbon), they were the lightest thing on the market, so I had to have one. It worked, it was light and not outrageously expensive, so I quickly bought another (alloy)
They work, they're light, but a big pain in the butt to adjust.
A seatpost's job is only to hold a saddle securely. It does that very well, yes. Mine has never
slipped. But like I said, it's REALLY difficult to change your saddle position [even slightly]. Maybe it's better since they got rid of the two bolt clamp... I dunno.
The problem is the design itself. The rail clamps don't bolt down directly perpendicular to the rails but PARALLEL to them.
Even changing the fore-aft position is more difficult than it should be; you have to release clamp pressure, which in so doing, sometimes affects the tilt.
But the real problem comes when adjusting or fine tuning the saddle tilt itself. Theoretically, it should provide infiinite adjustment. The reality is somewhat different. You have to totally back off the clamp pressure, which in so doing changes the angle of the saddle dramatically. So you have to start with the saddle in a very arbitrary nose down position, then tighten the clamp, which brings it up more or less level. It's a completely trial and error experience. Because the bolt has such a fine thread, it takes a few minutes to make one adjustment, so if you've done it wrong, you have to spend more time undoing it. But then the clamp sticks in place. How do you release the clamp? By banging the bolt heads sharply with a small hammer. But that stuffs up the most recent position. Even if you had it close, it'll be way off now!!! So you quickly get weary of making these continual ball-park adjustments.
I recently found that if you grease the hole clamp head, get the saddle level, then maintain downwards pressure on the nose of the saddle as your're tightening the rail clamps, it stays more or less in the same position.
Needless to say, I'm selling my alloy alien post (as part of a bike).