I use Obliterators quite a bit in my army. For me they are the ultimate tactical unit, capable of targeting any enemy unit and having a great chance at neutralizing it with shooting, and being decently armed for close combat to boot. Add on top of that excellent survivability and deep strike options all for a decent price and its almost a no-brainer as an elites choice.
Another reason why obliterators often make an appearance in my army lists is because I don't have a lot of choice for elite slots. In fact, I have two: obliterators or Chosen. All the cult troops (Berserkers, Thousand Sons, Plague Marines) are not valid options for me. So let's compare a 5 man terminator squad with Mark of Chaos Undivided, squad leader, autocannon, and a smattering of power weapons and power fist (about 233 pts) to a squad of 3 obliterators (210 pts):
- 6 wounds at toughness 4(5)
- 3 heavy weapons of varying type
- 2+(5+) save
- Deep Strike, no upgrades, Slow and Purposeful, and Fearless
- 5 wounds at toughness 4
- 1 heavy weapon
- 2+(5+) save
- Deep Strike, normal movement, normal leadership with re-roll
For less points the Obliterators have more wounds at a higher toughness and pack a more powerful heavy weapon punch. The terminators may have an advantage in close combat and move a bit faster, but otherwise the Obliterators are a superior choice.
(As an aside, don't bother bringing up Chosen in power armour as a valid third choice. The only valid reasons for taking a Chosen in power armour squad over a normal Marine squad are totally offset by their expensive costs. Who cares if I can get three heavy weapons in a squad of Chosen? You have to pay +3 points per model to get the Chosen and then pay Havoc squad prices for the weapons! Why not just take the freaking Havoc and use the points save for the models to get a fourth weapon? No, the only good reason to take chosen in power armour is as a bodyguard for the chaos lord so that they can come in from reserve together. And in that case... they count as HQ.)