My first and primary character in Path of Exile has been a witch that does damage by summoning an army of friends to do the attacking. This meant I got to ignore most of the stats on gear (damage, crit, accuracy, life leech...) because they all have no game effect. I want to make the enemies drop more loot and be harder to kill! More recently I've been playing a ranger and actually killing things for myself. This has opened up both life and mana leech as things I need to worry about since I'm attacking 5 times a second and am incredibly fragile.
Two of the stats you can get on gear are X% of physical attack damage leeched as life and X% of physical attack damage leeched as mana. These sound like pretty straightforward abilities. I expect that if I have 2% physical mana leech and shoot an enemy with an arrow that does 170 physical damage that I'd instantly recover 3.4 mana. Attack for 17000 physical damage and I expect to get healed for 340 mana. My ability only costs 25 mana so I don't need nearly that much mana leech but it sounds pretty sweet!
Unfortunately it is too sweet to be true. They put in caps on leeching to restrict how fast you could get life and mana back. That 17000 damage attack that restores 340 mana can actually only restore 12.5% of my maximum mana per second. I have 378 max mana so I can only get back 47.25 mana per second regardless of how much mana leech and physical damage I may have. This is a problem because I spend 125 mana per second attacking! It doesn't matter how much damage I do or how much mana leech I put on. In order to get more mana leech I actually need to increase my maximum mana pool! (Alternatively a high quality mana leech gem will increase the maximum rate of mana leeched. I have a 13% one of those, so my lightning arrow has a leech cap of 53 mana per second instead of 47.25. Still not nearly enough.)
Life leech is capped the same way except there the cap is 20% of maximum life and I have other incentives to increase my maximum life pool. It's still a very real cap (I can only get back 226 life per second right now) and is a real problem in terms of staying alive. I don't do much damage (2774 DPS with frenzy) but I should be leeching 270 life per second. So even with my mediocre damage I don't get the full impact of life leech.
There is an option here for life leech that changes things up a little. One of the skill nodes in the tree is called Vaal Pact and causes all of your life leech to happen instantly instead of being spread out like normal. This comes with the downside of preventing all other sources of life regen, including flasks. This still seems awesome though! It's an actual solution to reflecting monsters (even if you'd normally life leech enough to offset the reflected damage the reflected damage in instant and your leech is over time) and it removes the crazy cap which means adding more life leech or more damage will both help me out as opposed to right now where neither one will cause more life to get leeched.
Mana leech is still going to be a problem. Possible solutions are to switch to blood magic for my attacks, or to get mana regen from another source (clarity maybe?), or to decrease the cost of my attacks, or to increase my mana pool which will allow for more leeching to happen. There are a few really good max mana nodes spread out over the place and I may be able to fit those into a built. Or I could just get some more max mana on gear?