Lifegain has one big drawback that makes most pure lifegain cards (i.e. cards that only gain life, not gain life and do something else at the same time) near unplayable.

The problem is Magic is about getting your opponent to 0 life. Lifegain doesn't help accomplish this in any way. Lifegain isn't really played in any aggressive decks because it is probably the least aggresive thing you can do in Magic. Lifegain doesn't even stop your opponent from doing things (unlike cards that make your opponent discard, for example) so it's a very defensive play. It's basically saying you don't think your deck can win with the initial 20 life.

That's not saying lifegain is all bad. One of my favourite cards is Lightning Helix, which for 2 mana deals 3 damage to target creature or player and gains you 3 life, which is an excellent 1-2. They can kill your opponent or get an annoying creature out of the way, and the lifegain is a nice little bonus.

The other kind of lifegain that is somewhat playable are cards that gain you massive amounts of life for very little mana (like Martyr of the Sands for instance where you reveal X white cards from your hand and sacrifice Martyr of the Sands to gain 3 x X life - this is a possible 15 life on turn 2!) If you have reanimator decks, you can bring the Martyr back to life and gain 15 life per turn, which means most damage decks won't be able to kill you.

So, lifegain can be good, but most of the time it is not. Any pure lifgain card that costs 5 mana or more probably isn't worth playing. They can be fun in casual play, but not in serious or tournament play.

Sorry about the length of this response - I hope it makes sense