But are there any decks where cards whose primary purpose is life gain really shine? In what type of decks are they significantly better or worse than other types of cards? What combos or strategies upgrade them from average cards to truly great ones? How can I tell if I should add some life gain cards to my deck?
Respectively: no; they're always worse; none, and anyway they were terrible cards to begin with; you shouldn't. ;-)
Seriously though, assuming you're talking about constructed, cards whose only purpose is to gain life contribute nothing useful to any deck that aims to be anything more than purely casual. Think about the possible win conditions you could be playing against:
- Creature beatdown: if you gain some life, the creatures will take it right back, and then you're in exactly the same situation you were before - in other words, your life gain spell effectively makes you skip your turn
- Poison counters: life doesn't matter
- Mill: again, life doesn't matter
- Combo: life doesn't matter because when a combo deck goes off, it can usually deal a huge amount of damage in one turn, so no matter how much life you have, you're still toast
Basically, life gain spells use up resources (mana, cards) that could be devoted to other spells which would help your game situation more.
The only place you could argue that life gain cards might fit in is when you can exchange that extra life for some more important resource. The original example was Necropotence: each life you gain becomes a card you can draw, so Stream of Life acts like Braingeyser (which is ridiculously powerful, FYI). More recently, you have Felidar Sovereign as Shane mentioned, which means that gaining life brings you closer to winning in the same way that dealing damage to your opponent does. In that case, Stream of Life acts like Red Sun's Zenith (although it only works on players, which kind of holds it back because being able to target a creature is the main appeal of direct damage).
Even in decks that make use of life in this way, though, pure life gain spells aren't usually the best choice, because there are other, better spells that gain you life as a side effect of doing something else. In the Necro deck, it was Drain Life, which dealt damage to any creature or player (great deal) and gained you life as a side effect. In combination with Necropotence, it acted like Red Sun's Zenith and Braingeyser in the same card, which explains why Necro was arguably the most dominant deck in Magic history. In a Felidar Sovereign deck, you would have lifelink creatures, which would again gain you life as a side effect of dealing damage.
There is one case that I know of in which a pure life-gain effect (of sorts) achieved some success, and that would be Soul Sisters, the deck centered around Soul Warden and, in a later incarnation, Suture Priest. Even so, the only reason the deck worked was that it included a bunch of other cards that got pretty decent bonuses off gains in life, like Serra Ascendant and Ajani's Pridemate. So the life gain was just a roundabout way to pump up the creatures. The later version of the deck did use the combination of Leonin Relic-Warder and Phyrexian Metamorph which could gain an arbitrarily large amount of life, but even that was only a stalling measure, and only worthwhile because the cards involved had other practical uses.