It's pretty hard to appreciate. It gives a wide variety of stats that aren't terribly strong on their own (100% mana regen, 40 AP, 10% cdr, 8% move speed) but otherwise no defense, and is just barely gold efficient (105.84% gold efficiency). Then there's the passive that is hard to appreciate (it gives about 1050g worth of free stats to each ally. Even if it's just the adc benefiting from it, it's still 50% more gold efficient.)
That said, it's definitely a niche item, but is a very good one on some champions (Soraka, Sona). It makes sieging a turret considerably easier, especially as a team. Team fights, it's pretty useful for your ADC/other AA-reliant champions. It's basically core on Sona, who uses all the stats, and applies enhanced shields to her team easily (and with 40CDR, basically permanently)
*I say 1050g worth of stats because it's assumed that 1 on-hit damage=20g. I mention Sona because, seriously, it is extremely good on her. It's not on Sona's highest popular/winrate builds in plat+ for nothing.