ok most people who start out playing magic think life gain is something realy powerful and like it a lot. later on you learn that lifegain is not very good. both points of view are not fully correct. if you want to know the whole story behind lifegain read on ...
lifegain has one big drawback that leads to most lifegain cards being awful. lifegain does not kill anyone. a card that only gains life has no place in your average agressive deck because it is one of the least agressive things you could do. since lifegain also does not stop the opponent from doing relavant things its also not that good in control where counter or discard etc. are much better. in combo you will only want to put together your combo and win. a card that only gains life will hinder your own plans and thus is utter wrong in this kind of deck.
ok but why is lifegain not all bad then ? well as such life gain is very nice. it refils one of your most crucial recources, your life. however the biggest portion of straight lifegain cards simply do too little. there are 2 very important exceptions to this:
1. lifegain + x
ok cards that gain you life but do something very relavant at the same time are one of the easyest ways to make lifegain good Lightning Helix is a wonderful example of this. Baneslayer Angel is another very good example. both can help you kill your opponent get rid of creature or block them and at the same time gain you life. your spending your recouces on something that furtheres your gameplan in a relavant way and gains you life on top. now thats very good life gain.
2. lifegain > x
ok so cards that only gain life are bad right ? not quite. if the life they gain you are enough for a small enough cost they can actualy start bekomming interesting. also when the can gain you life over and over thats quite interesting.
so whats enough life then ? i'd say at least 10. that means your avarege agro deck will need 1-2 turns longer to kill you. you will have more chances to draw the wrath of god or something else that helps. since agro usualy kills you in 5 or less tuns any lifegain that costs more than 4 is just too slow. also the cheaper the better. one of the best in my oppinion is Martyr of Sands thats usualy 12-15 life on turn 2 ... now that does slow agro down a lot.
the martyr also brings me to the second type. repeatable lifegain. so what if i revive the matyr every turn and make 15 life every turn? you will be very hard to kill by damage yes. there have even been and are tournament decks that do just that. gain huge amounts of life over and over again. in most of them the martyr is the lifegainer. smaller amount of repeated lifegain can also be good. the white honden gains you 2 life every turn. ok thats not nearly enough to slow agro but you can do other things with it. Zur's Weirding is a very powerful card if you can pay them 2 life every turn longer than your opponent. cards like these require only little but repeated lifegain to work.
so you see lifegain can be good. most of the time it is not. any card that costs 5 or more mana will most like not be good. they can still be fun for casual on occasions but they will make your decks less powerful by including them over something worth while.
if you like me are a big fan of the martyr after reading this let me know and i will tell you some of my favourite combos with that card ^^.