I use one on my Anthem X. I like it. Depending on the saddle they can be a bit punishing on a hardtail – unless the saddle has the FX-style beam then it’s all very solid indeed under the padding.
An I-Beam setup has loads of fore-aft adjustment. They’re note always incredibly easy to adjust, though. The clamp on mine has had a habit of binding to the top of the seat post and requiring a few whacks to loosen off, unless I was pretty assiduous when greasing them initially. I don’t recall mine every creaking.
A friend’s SDG I-Beam seat post and its head parted company on a very cold ride a few years ago. It seemed to be a case of the bonding between them being weakened due to the cold, or something like that. I don’t know if this has been fixed or not.
Personally I’d have I-Beams on more of my bikes if only someone produced an uppy-downy seatpost with an I-Beam head.