Riot gave Blade of the Ruined King (BotRK) a huge buff a couple of weeks ago. It took only two days for them to realize they overdid it, and they subsequently deployed a hotfix to address it. I think it got slightly overnerfed (needed the AD reduction or the price increase, but not both), but it's still a very good item. However, it shouldn't be blindly bought on anyone.
Things to Consider
When you're making your decision on whether or not to buy a Blade of the Ruined King, consider these important factors:
1) How much of your damage comes from your auto-attack and/or AP scaling attacks? The higher this percentage, the more attractive BotRK becomes. So characters like Tristana and Vayne are a lot more likely to want BotRK than carries like Urgot or Corki.
2) What's the health situation of your opponents? You can think about BotRK as giving you 25 AD and making your auto-attacks only have additional AD = 2.5% of the health of your target while making both 40% faster. A little bit of algebra tells us that it breaks even with Bloodthirster (BT) at roughly 2400 health for the target. This means that if the other team buys a couple Giant's Belts, BotRK becomes more attractive. If they don't, BT and/or Infinity Edge (IE) are the superior choices.
3) What's the armor situation of your opponents? Armor counters BotRK (and BT) fairly hard. If they're buying modest amounts of armor, Black Cleaver (BC) may be the right buy. If they're stacking armor significantly, Last Whisper (LW) is where it's at.
4) Are you seeing attack speed slows (i.e. Randuin's, Frozen Heart, etc)? In such an environment, Infinity Edge/Phantom Dancer are probably bigger priorities. It doesn't matter how slow you attack if your crits are still hitting for giant chunks. The attack speed debuff still hurts, but if your alpha is high enough, it doesn't matter as much.