Whirlwinds are seriously underrated. With cover so strong in 5th edition, denying it is a powerful ability. Being able to in cover yourself and fire, ignoring enemy cover gives them increased protection too.

I was actually advising a Space Wolf player last night to not take his 2 Whirlwinds in a Space Marine dominated metagame, but he killed 3 terminators over 2 turns with 1. Hitting several models at once is going to push some dice to land on those glorious 1s!

The pinning and AP makes them an excellent horde slayer. I often take down Ork lootas and Shokk guns with mine.