I probably run this card on Jnet more often than most people, and I'd just like to note one thing that the previous review overlooks. Hydra only costs 10 when it doesn't fire. Otherwise its net cost is only 5 because the second sub immediately pays you back half the cost (obviously you still need the full price on hand for the rez). This makes Hydra a surpisingly viable option as a midgame rez for an averagely rich deck. You can go broke to rez it and it will immediately pay you back enough cash to play a Hedge Fund. Meanwhile, clearing their tag costs the runner a click they might otherwise have used to capitalise on your low funds. All this means that as long as you're sure the runner is going to bounce off it, Hydra can be favourably compared to mid value ice, whether it's stoppers like Mestnichestvo or tax-walls like Funhouse. You could even argue it's preferable to Tollbooth, in certain lights, if you squint a bit.
In the case the runner can break it, the subs are irrelevant, so all you need to know is the numbers. In this case it's still not bad if you can afford it (only one strength less than Týr and without the uniqueness or alternate breaking option). This is an occasionally handy but - in my mind - secondary use. Combined with the analysis above, you might think of Hydra as a very good medium-cost ice that becomes a slightly-below-the-curve high-cost ice once the runner can break it - whether you've rezzed it by that point or not.
All this means that in its optimum use-case it's a very code gate-like sentry (note all the comparisons above are code gates). It doesn't do the runner really dangerous damage when it hits, but it creates a big economic tempo swing. You get: a big ETR ice for the price of a middle-sized ice. They get: a tag. In its less-than-optimum use case it's just some big numbers.