Like most people, I got Riven as she was released. Riven was my first melee DPS character that I actually do well with. And the reason for this, I believe, is that I build her very differently than most other melee DPS characters.

TL;DR of what you are about to read: I believe that in the early/mid game, for a melee character, the best way to sustain yourself in a lane is through HP/5 items rather than buying life-steal.

I have a somewhat strange build for Riven. It involves getting Tiamat and Philosopher's Stone. And I don't get any life-steal until I am getting a Bloodthirster in the late game. Here is why.

As a melee character, you have to be very close to the enemy minions to hit them. The problem with this is, by being close to the enemy minions, you are within easy striking range of the enemy. Eventually you will enter a cycle of jump in, last hit, hit each other a little, and then back off.

To illustrate this, I'll give you an example of when I was in the solo top lane against another riven, one who followed the more common riven build of rushing brutalizer and then getting a vampiric scepter for sustain. I'll focus on one point in the game.

She saw me in the loading screen and saw that i was the only champ on the team capable of effectively going top lane. So to counter me she bought a doran's shield. Its a bit later in the game so she also has her CDR boots, a brutalizer, a vampiric scepter, and her doran's shield. I have my philostone complete, my tiamat complete, and level 1 boots. We are both around level 9.

I have 150 damage, she has 120. She has more armor, and way more CDR. If we were to enter a straight fight, she would win. We both have exhaust and flash.

So what I do is whenever she comes close, I dash in, stun her, and run back a little. Of course, she hits me back with a bit of damage, and this goes on with us taking about equal amounts of damage. But I make sure to not let her stay on my minions. Because while she can beat me in a straight fight, she can't win if my minions are with me.

But here is where it gets interesting. About every 6 seconds she will come in for a last hit. She procs her passive in some way every time she does, and she restores 15 health through lifesteal every last-hit. She has 29 hp/5. This means that on average she will heal herself for 40-45 every 5 seconds, unless the wave is under my tower, at which point she will only heal for 29 every 5 seconds.

I have my base 21 hp/5 at level 9, and +33 hp/5 from my items. this means that every 5 seconds I have a guaranteed 54 HP heal. As you can see, I have more ability to regenerate than her.

I get in a fight with her in the bush closest to my tower. I dash in and initiate a fight. I know that I will lose, but I'm close enough to my tower to escape safely. Our combo attacks are going off, and we are locked in a fierce battle. I escape with about 1/8 of my HP left and the enemy Riven is at 1/4 HP remaining. Neither of us farm much because we could easily burst eachother down, and don't want to get too close to each other.

So, she isn't last hitting much. She just has her 29 hp/5, meanwhile I am getting health back twice as fast as her. Before she knows it, I am at half health while she is still at 3/8. I proceed to initiate more fights, and continue to gain an HP advantage over her by regening health faster.

I proceed to zone her out hard and win the lane because I could get health back much faster than she could, even though she is stronger than me.

But that's just an example of why health regen can help you win the lane by recovering from a close fight much faster.

Of course, if the enemy is way stronger than you are, the benefits of HP/5 becomes much greater. I don't have a personal example here, so this is just theory, but if the enemy is clearly stronger than you, then this can let you stay in the lane longer without getting low enough to die, while having life-steal would not benefit you do to you barely attacking minions at all.

Now, if you are a ranged AD character, then life-steal is better because you don't have to go as far into harms way to last-hit. But for all the Rivens, Tryndameres , Yis , AD Sions, and Gangplancks who don't have a ranged attack, you would benefit more from Hp/5 than life-steal.
Now, this is just for the early-mid laning/ganking phase. But when you get to the late game where you are doing 200-250 damage a hit and could easily just visit a creep camp, life steal would benefit you more.