Numbers relating to the first small Siphon Unit are up in a Dev blog.
I don’t know enough about what they are designed to steal to understand where they might be profitable, and how long it will take to get back your 10M ISK investment. You can’t shift them once anchored, so they are there until shot down or (I assume) left un-accessed for long enough to expire.
It’s an interesting mechanic.
I could imagine it impacting those large Alliances with cheery picked moons spread across multiple regions, that they only visit once a week or so. This seems a good thing.
For POS in your own Sovereignty space, you might just need to do a D-Scan once a day in each system to find if any have been anchored, clean out what they have stolen, and then remove them. This would be easier for those who live out of a constellation compared to those who own multiple regions. This also seems a good thing.
For smaller operators who live and work around their POS they should be able to notice and address them fairly quickly.
For those more combative groups in nature, and given you can anchor more than one on each POS, it could be used to wage economic warfare, or even provide an income source. (So maybe it can be put down as a form of tax on an area of Low Sec / NPC 0.0 you are able to partially control.)
As I said – an interesting mechanic that adds more dynamics to the sand pit.
So how do they work out for the Solo Player?
They are small and easily anchored, so available for anyone to make use of from a solo capacity. You might annoy your neighbours or some far away large alliance, but it is not as if they wouldn’t kill you at any opportunity you gave them anyway.
The issue will be for the small POS operators in Low Sec and NPC 0.0 who already have little margin on their endeavours (which is why they were able to use those moons without someone bigger kicking them off in the first place). Having said that, if you live around your POS you should be able to identify these quicker than the average group, and use some anchored POS weapons to remove them safely.
Not ideal maybe, and certainly an annoyance, but not horrible for the average solo player. In fact, the more active you are in your local space, the less negatives there are from these things.