I have two of these 49ers from Fasttech, the £9 one mentioned above and the slightly more expensive "1:1" version. I'm not a new vaper by any means (five years!) but it did take me a while to suss them out.
The "spacer" that people have been going on about is not a spacer but an integral part of the button mechanism - the magnetic resistance working in place of a spring.
I've found that, contrary to what you might have read, flat-top batteries work best I had mixed results with button tops – I had a couple that got jammed in the atty hole on the cheaper clone - alarming fireworks a plenty! – but at least I can confirm that it does indeed vent (spectacularly!)
The throw can be adjusted by adding magnets to the base of the spacer. I have a dozen
8x1mm earth magnets and use 1, 2, or sometimes three stacked depending on the battery. I like as short a throw as possible as I find having to press the recessed button more than a couple of mm a little uncomfortable.
The button on the £9 version is intially rather crunchy, the 1:1 far less so, however in both instances this improves with use. The breaking in can be accelerated with a gentle bit of wet fine grade sanding and performance improved with a bit of graphite on the edges (you can use a pencil).
I haven't had any atty problems with the 1:1 – they all seem to fit. The cheaper clone is a bit more selective.
On the whole I'm very pleased with these mods – they're tiny powerhouses and look gorgeous – however I wouldn't recommend them to anyone new to mechanicals or people who like their mods to be plug-in-and-play ready.